materials Review

Hole-Transporting Materials for Printable Perovskite Solar Cells Paola Vivo *

ID

, Jagadish K. Salunke and Arri Priimagi

Laboratory of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, FI-33101 Tampere, Finland; [email protected] (J.K.S.); [email protected] (A.P.) * Correspondence: [email protected]; Tel.: +358-44-340-7081 Received: 19 August 2017; Accepted: 12 September 2017; Published: 15 September 2017

Abstract: Perovskite solar cells (PSCs) represent undoubtedly the most significant breakthrough in photovoltaic technology since the 1970s, with an increase in their power conversion efficiency from less than 5% to over 22% in just a few years. Hole-transporting materials (HTMs) are an essential building block of PSC architectures. Currently, 2,20 ,7,70 -tetrakis-(N,N0 -di-p-methoxyphenylamine)9,90 -spirobifluorene), better known as spiro-OMeTAD, is the most widely-used HTM to obtain high-efficiency devices. However, it is a tremendously expensive material with mediocre hole carrier mobility. To ensure wide-scale application of PSC-based technologies, alternative HTMs are being proposed. Solution-processable HTMs are crucial to develop inexpensive, high-throughput and printable large-area PSCs. In this review, we present the most recent advances in the design and development of different types of HTMs, with a particular focus on mesoscopic PSCs. Finally, we outline possible future research directions for further optimization of the HTMs to achieve low-cost, stable and large-area PSCs. Keywords: perovskite solar cells; hole-transporting material; printable; small-molecule; polymer; inorganic; hybrid

1. Introduction The world demand for energy is growing rapidly and continuously. In 2016, the total worldwide energy consumption was approximately 1.33 × 108 tonnes of oil equivalent (toe) [1], which corresponds to roughly 1.5 × 105 terawatt hours (TWh). These numbers continuously rise, due to the growth of the population and the world economy. Hence, there is no other clever option to meet the future energy demands than to invest in environmentally-clean energy resources, such as solar energy. Every year the Sun provides the Earth’s surface with 1.9 × 108 TWh of radiation [2]; thus, it supplies in about 7 h enough energy to fulfill the world energy needs for one year. Sunlight is free and unlimited. Moreover, solar energy is a clean, renewable, readily available and ubiquitous energy source. The large gap between the current use of solar energy and its unexploited potential represents a great challenge in energy research. Conventional crystalline silicon-based solar cells are present in still 90% of commercial photovoltaic devices. However, there is a need to replace silicon photovoltaics (PV) with low-cost, easy-to-assemble, flexible and lightweight devices. Recently, hybrid organic-inorganic perovskite solar cells (PSCs) became in just a few years one of the most exciting PV technologies. They have a huge potential to dominate the photovoltaic market, being at the same time highly efficient, low-cost and compatible with inexpensive fabrication processes such as inkjet printing. However, despite the impressive advances in their power conversion efficiencies over the last few years, PSCs still suffer major stability issues (lifetime 10−3 cm2 V−1 s−1 ), as well as thermal and photochemical stability are required characteristics of an HTM [5]. Furthermore, transparency in the visible spectrum is desirable to avoid the absorption screen effect toward the active materials/absorber. In order to avoid crystallization, an amorphous phase with a glass transition temperature above 100 ◦ C is also required. In the case of the mesoscopic architecture, adequate HTM pore filling is needed, as well. This can be more easily obtained with small-molecule HTMs. Finally, the thickness optimization of the HTM layer is also important to minimize the increase in the series resistance, which directly correlates with the cell fill factor (FF) reduction. Usually, an optimal thickness of the HTM lies within the 100–200 nm range. The most widely-explored compounds for HTM applications are certainly those based on the triphenylamine (TPA) moiety. In particular, 2,20 ,7,70 -tetrakis-(N,N0 -di-p-methoxyphenylamine)-9,90 spirobifluorene), or spiro-OMeTAD, is a TPA-based HTM adopted in the great majority of the works, first on solid-state DSSCs [38] and later on PSCs, as well (Figure 4). The reason why spiro-OMeTAD is still the most widely-used HTM is that it leads to very efficient PSCs. The PCEs achieved with spiro-OMeTAD have rarely been achieved or exceeded with other HTMs.

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Figure 4. Molecular widely-used HTM in in PSCs, andand its Molecularstructures structuresofofspiro-OMeTAD, spiro-OMeTAD,thethemost most widely-used HTM PSCs, dopants (lithium bis(trifluoromethanesulfonyl) its dopants (lithium bis(trifluoromethanesulfonyl)imide imidesalt salt(LiTFSI), (LiTFSI), 4-tert-butylpyridine 4-tert-butylpyridine (TBP), tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) tri[bis(trifluoromethane)sulfonimide] tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) tri[bis(trifluoromethane)sulfonimide](FK (FK209). 209).

Nevertheless, there are serious drawbacks of spiro-OMeTAD that preclude it from being the Nevertheless, there are serious drawbacks of spiro-OMeTAD that preclude it from being the definitive HTM in PSCs, such as: definitive HTM in PSCs, such as:  high costs: spiro-OMeTAD is prohibitively expensive (~500 $/g) [39] because of (a) its onerous • high costs: spiro-OMeTAD is prohibitively expensive (~500 $/g) [39] because of (a) its onerous multistep synthesis that requires a low temperature (−78 °C); (b) the sensitive (n-butyllithium or multistep synthesis that requires a low temperature (−78 ◦ C); (b) the sensitive (n-butyllithium or Grignard reagents) and aggressive (Br2) reagents involved in the synthetic scheme [40]; (c) the Grignard reagents) and aggressive (Br2 ) reagents involved in the synthetic scheme [40]; (c) the costly sublimation step required for purification [41]. costly sublimation step required for purification [41].  negative impact on stability: the use of spiro-OMeTAD limits the long-term stability of the negative impact on stability: the use of spiro-OMeTAD limits the long-term stability of the • devices [39], thus inhibiting the upscaling application in the photovoltaic industry. devices [39], thus inhibiting the upscaling application in the photovoltaic industry.  sub-optimal charge transport: when in pristine form, spiro-OMeTAD shows a modest • sub-optimal charge transport: when in pristine form, shows a modest hole-mobility 2 V−1spiro-OMeTAD −7 S cm −1, respectively) hole-mobility and conductivity (1.67 × 10−5 cm s−1 and 3.54 × 10 [42], −5 cm2 V−1 s−1 and 3.54 × 10−7 S cm−1 , respectively) [42], thus and conductivity (1.67 × 10 thus requiring additives and chemical p-dopants to enhance the hole conductivity by 10-fold requiring additives and chemical p-dopants and thus the conversion efficiency of PSCs. to enhance the hole conductivity by 10-fold and thus the conversion efficiency of PSCs. The dopants typically incorporated into spiro-OMeTAD solution are lithium The dopants typicallyimide incorporated into4-tert-butylpyridine spiro-OMeTAD (TBP) solution are cobalt lithium bis(trifluoromethanesulfonyl) salt (LiTFSI) [43], [43], and (III) bis(trifluoromethanesulfonyl) imide salt (LiTFSI) [43], 4-tert-butylpyridine (TBP) [43], and cobalt (III) complexes such as tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) tri[bis(trifluoromethane) complexes such as tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) sulfonimide] (Co[t-BuPyPz]3[TFSI]3), coded as FK 209 [44]. Their moleculartri[bis(trifluoromethane) structures are shown in sulfonimide] (Co[t-BuPyPz]3[TFSI]3), coded 209 [44]. molecular structures are shown Figure 4. While on the one hand, theas FK dopants areTheir crucial for the enhancement of in Figure 4. While on the one in hand, theofdopants are crucial forbetter the enhancement of spiro-OMeTAD spiro-OMeTAD performance terms higher conductivity, electron injection and retarded performance in on terms of higher conductivity, electron injection and retarded recombination, recombination, the other hand, their use canbetter negatively impact the stability of the devices [45]. on the hand, can negatively impact stability of the [45]. to spiro-OMeTAD Inother the last fewtheir years,use a wide range of HTMs hasthe been proposed asdevices alternatives In the last few a wide and range of HTMscompounds has been proposed as alternatives to spiro-OMeTAD in PSCs, based bothyears, on organic inorganic [5,8]. However, only very few examples in PSCs, based both on organic and inorganic compounds [5,8]. However, only very few examples allowed reaching or surpassing the power conversion efficiency (PCE) of 20% [10,12,39,46,47], which allowed reaching powerwith conversion efficiency (PCE) Currently, of 20% [10,12,39,46,47], is comparable withorthesurpassing efficienciesthe obtained spiro-OMeTAD [48–50]. the champion which is comparable with the efficiencies obtained spiro-OMeTAD [48–50]. Currently, the PSC with world record efficiency includes thewith polymeric HTM poly-[bis(4-phenyl)(2,4,6champion PSC with world record[12]. efficiency includes the polymeric HTM poly-[bis(4-phenyl)(2,4,6trimethylphenyl)amine] (PTAA) trimethylphenyl)amine] [12].the work by Mei et al., which contains one of the rare examples It is also important (PTAA) to mention is also important to mention the[51]. work Mei et al., which rareabsorber examples of of anItHTM-free perovskite solar cell Inby fact, perovskite cancontains functionone as of thethe light and an HTM-free perovskite solar cell [51]. fact, perovskite function theHTM-free light absorber and hole hole transporter, as well. Despite the In modest efficiency can achieved byasthe configuration transporter, as well. Despite the modest efficiency achieved by the HTM-free configuration (12.8%), the work reported in [51] presents a highly stable solar cell structure able to last over(12.8%), 1000 h the work reported in [51] presents a highly stable solar cell structure able last over 1000 harchitecture in ambient in ambient conditions under full sunlight illumination. The heart of thistofully-printable is a double layer of mesoporous ZrO2 and TiO2 scaffold infiltrated with perovskite, which allows low defect concentration in the perovskite crystallization, as well as better contact with the TiO2 scaffold.

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conditions under full sunlight illumination. The heart of this fully-printable architecture is a double layer of mesoporous ZrO2 and TiO2 scaffold infiltrated with perovskite, which allows low defect concentration in the perovskite crystallization, as well as better contact with the TiO2 scaffold. In this section, the most interesting examples for each category of HTMs, namely organic, inorganic and hybrid, are reviewed. However, due to the extremely intense research on the topic, it will not be possible to present all of the numerous works on HTMs, but only a relatively small selection of them. From this section, the reader can have a picture of different classes of HTMs, among those which are solution-processable and thus suitable for printable solar cells. It is worth noting that a straightforward comparison between the efficiencies of the many HTMs presented is not possible, as the cell performances are closely related to many factors (e.g., operating conditions, optimization of the other constituents of the PSC, measurements’ accuracy, etc.), which vary from lab to lab. That is why in all of the examples, it has been always important to compare the figure of merits of the devices based on the target HTM with those of the control cells containing spiro-OMeTAD. Furthermore, very recently, mixed-ion formamidinium lead iodide—methyl ammonium lead bromide (FAPbI3 )1−x (MAPbBr3 ) perovskites have been introduced, with a significant boost in the device PCEs. Hence, since most of the reported HTMs have not been tested yet with the mixed-ion perovskites, an objective comparison among different HTMs is not currently possible. 3.1. Organic Hole-Transporting Materials Organic hole-transporting materials can be classified into small-molecule-based, polymer-based and organometallic-complex-based HTMs. We present an overview of the most recent and promising works in view of attaining efficient, inexpensive and stable PSCs. Since the organometallic compounds, like phthalocyanines, still perform relatively poorly compared to other organic HTMs in PSC, we will not describe these systems in the present work. The interested reader can refer to, e.g., [5,52,53]. 3.1.1. Small-Molecule-Based HTMs This category comprises the largest number of newly-designed HTMs for PSC research. Small molecules are suitable when thinking of PSC technology upscaling, because they have a distinct structure and molecular weight and, thus, can be easily reproduced for industrial production with high purity and high yield [39]. Moreover, small organic molecules are superior to conjugated polymers from the viewpoints of the easier synthesis process, better reproducibility, relatively easy tuning of the optical and electrochemical properties by changing different functionalities, low molecular weight and solution processability. Most small-molecule-based HTMs contain nitrogen and sulfur, which are electron-rich atoms and, thus, particularly suitable for HTMs. They are usually based on a triphenylamine (TPA) moiety, due to the presence of the electron-rich nitrogen atom, which minimizes the intermolecular distance, leading to non-planarity of the TPA system. This ultimately results in the formation of amorphous materials, a beneficial feature for HTMs because of their capability to form smooth and pinhole-free films. This ensures uniform contact at the interface with the metal electrodes [54]. However, as mentioned earlier, the amorphous nature of these materials imparts poor hole-mobility. Such a drawback can be overcome by using different organic and hybrid dopants (Figure 4). Nevertheless, one issue regarding the use of dopants is their hygroscopic nature, which reduces their stability and ultimately affects the degradation of the PSCs. The discovery of new stable dopants is an open issue for molecular designers, the discussion of which goes beyond the scope of this article. Herein, we classify the different small-molecule-based HTMs based on their chemical composition and give a summary of the photovoltaic performance of the presented HTMs (Table 1) as compared to a reference cell containing spiro-OMeTAD.

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Table 1. Summary of the photovoltaic performance (PCE) for each of the small-molecule HTMs reviewed in this work. For comparison, the PCE of the corresponding reference cells containing spiro-OMeTAD (fabricated and characterized in identical conditions) is also reported. TPA, triphenylamine. Category

HTM

PCE (%), HTM

PCE (%), Spiro-OMeTAD

Reference

Pyrene-based

PY-1 PY-2 PY-3

3.3 12.3 12.4

12.70 12.70 12.70

[55] [55] [55]

Truxene-core

Trux-I Trux-II Triazatrux-I Triazatrux-II Triazatrux-III Triazatrux-IV Triazatrux-V Triazatrux-VI Triazatrux-VII

18.6 (10.2 [56]) 13.4 8.9 17.7 15.8 11.5 8.88 14.87 19.03

16.0 (9.5 [56]) 9.50 17.10 17.10 17.10 17.10 19.01 19.01 19.01

[57] [56] [58] [58] [58] [58] [59] [59] [59]

Phenothiazine-based

PH-I PH-II

2.10 17.60

17.70 17.70

[60] [60]

Acridine-, thiophene-, biphenyl-, bithiophene-, tetrathiophene-, difluorobenzene, and phenyl-based

AC-I Thio-I Thio-II BPH-I BPH-II BTHIO TETRATH-I TETRATH-II TETRATH-III TETRATH-IV DFTAB

16.42 9.05 15.13 13.27 16.42 19.40 18.13 17.3 15.7 9.7 10.4

16.26 8.83 (15.63) 1 8.83 (15.63) 1 16.81 16.81 18.80 17.80 17.80 17.80 17.80 15

[61] [62] [62] [63] [63] [64] [65] [65] [65] [65] [66]

Triazine-based

TRIAZ-I TRIAZ-II TRIAZ-III TRIAZ-IV

12.5 10.9 13.2 12.6

13.45 13.45 13.8 13.8

[67] [67] [68] [68]

Benzotrithiopheneand squaraine-based

BZTR-I BZTR-II BZTR-III BZTR-IV BZTR-VHYX SQ-H SQ-OC6 H13

16 17 18.2 19 18.2 14.74 14.73

18.1 18.1 18.1 18.9 18.9 15.33 15.33

[69] [69] [69] [70] [70] [71] [71]

Fluorene- and spiro-fluorene-based

FL-I FL-III FL-IV FL-V FL-VI FL-VII FL-VIII FL-IX Spiro-FL-I Spiro-FL-II Spiro-FL-III XDB XOP XMP XPP FDT SPI-BI

17.8 17.25 15.90 14.52 15.09 9.15 16.79 16.45 19.8 13.6 20.8 5.4 15.0 16.5 17.2 20.2 17.77

18.4 16.67 16.67 17.88 17.88 17.88 17.88 17.88 20.8 18.8 18.8 5.5 5.5 5.5 5.5 19.7 18.25

[72] [74] [74] [75] [75] [75] [75] [75] [76] [77] [77] [78] [78] [78] [78] [39] [79]

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Table 1. Cont. Category

HTM

PCE (%), Spiro-OMeTAD

Reference

FL-II SPI-TH SPI-TRI SPI-FL-MM-3PA SPI-FL-MP-3PA SPI-FL-MM-2PA SPI-FL-MP-2PA

16.73 19.96 19.47 13.46 15.59 12.56 13.43

14.84 18.25 18.25 14.98 14.98 14.98 14.98

[73] [79] [79] [80] [80] [80] [80]

Carbazole-based

CA-I CA-II CA-III CA-IV CA-V CA-VI CA-VII CA-VIII CA-IX CA-X CA-XI CA-XII CA-XIII CA-XIV CA-XV CA-XVI CA-XVII CA-XVIII CA-XIX CA-XX CA-XXI CA-XXII CA-XXIII

12.3 8.5 10.2 13.28 14.79 13.86 4.53 0.19 7.6 9.8 10.96 12.61 13.0 11.4 13.1 17.8 17.81 12.42 14.92 16.74 0 13.30 16.87

12.17 10.2 10.2 15.23 15.23 15.23 5.10 5.10 10.2 10.2 13.76 13.76 13.76 12.0 12.0 18.6 18.59 14.32 15.01 15.01 15.01 15.01 15.53

[81] [82] [82] [83] [83] [83] [84] [84] [85] [85] [86] [86] [86] [87] [87] [88] [89] [90] [91] [91] [91] [91] [92]

Other small molecules (Naphthalene (NPH), di- and tetra-phenylmethane (DPA-TPM, TPA-TPM), and ethylene dioxythiophene (EDOT))

NPH-I NPH-II OMe-I OMe-II Thiazo-I Thiazo-II Thiazo-III Ph-TPM DPA-TPM TPA-TPM EDOT-AZO

10.05 8.66 18.34 16.14 10.60 4.37 8.63 4.62 9.33 15.06 11.0

10.06 10.06 15.49 15.49 15.49 11.9

[93] [93] [94] [94] [95] [95] [95] [96] [96] [96] [97]

1

(a)

PCE (%), HTM

When the spiro-OMeTAD concentration is increased up to 73 mg mL−1 .

Pyrene-based HTMs

One of the first works on alternative HTMs appeared in 2013, when Seok et al. proposed a set of pyrene-core arylamine derivative HTMs, with a performance comparable to that of spiro-OMeTAD [55]. In these molecules, the spirobifluorene core of spiro-OMeTAD is replaced by a pyrene core (PY-1, PY-2 and PY-3; Figure 5). Methoxy (-OCH3 ) groups are present also in these pyrene-based HTMs, though their position is changed from para (as in each of the quadrants of spiro-OMeTAD) to meta or ortho. In fact, the -OCH3 groups play an important role not only in controlling the electronic properties of spiro-OMeTAD by adjusting the HOMO levels of the materials, but they are also responsible for anchoring the material onto the underlying perovskite layer [4,5]. The electron-donating effect of methoxy groups in the N,N-di-p-methoxy phenyl amine (X in Figure 5), which is directly bonded to the

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materials, but they are also responsible for anchoring the material onto the underlying perovskite layer [4,5]. The electron-donating effect of methoxy groups in the N,N-di-p-methoxy phenyl amine pyrene increases electron density. the electron density, both thedensity. HOMOBy and (X in moiety, Figure 5), which the is directly bonded to By theenhancing pyrene moiety, increases the electron the electron density,orbital both the HOMO and levels the lowest unoccupied molecular orbital theenhancing lowest unoccupied molecular (LUMO) energy (and thus, the band gap) are modified. (and thus, the band gap) are PY-1 modified. As a lower result,efficiency a high PCE of 12.4% As(LUMO) a result,energy a high levels PCE of 12.4% (PY-3) is achieved. exhibited (3.3%), which (PY-3) achieved. PY-1driving exhibited lower (3.3%),The which is due to the insufficient driving is due to isthe insufficient force forefficiency hole injection. well-known spiro-OMeTAD showed force for hole injection. well-known spiro-OMeTAD showed PCE of of 12.7% under similar PCE of 12.7% under similarThe fabrication conditions. The slightly higher PCE spiro-OMeTAD cells fabrication conditions. The slightly higher PCE of spiro-OMeTAD cells as compared to PY-3 ones as compared to PY-3 ones originates from the higher short-circuit current (JSC ), i.e., 21 mA cm−2 −2, achieved for higher short-circuit current ), i.e., 21 mA cm−2 vs. 20.2 mA cm(V 2 , achieved vs.originates 20.2 mAfrom cm−the for PY-3 cells, and(JSC the higher open-circuit voltage OC ), i.e., 1.01 V PY-3 cells, and the higher open-circuit voltage (V OC), i.e., 1.01 V (spiro-OMeTAD) vs. 0.93 V (PY-3). (spiro-OMeTAD) vs. 0.93 V (PY-3). This indicates more efficient charge collection via spiro-OMeTAD This indicates more efficient charge collection via spiro-OMeTAD HTM and better matching between HTM and better matching between the quasi-Fermi level of the electrons in TiO2 and the HOMO the quasi-Fermi level of the electrons in TiO2 and the HOMO of spiro-OMeTAD. Conversely, a higher of spiro-OMeTAD. Conversely, a higher fill factor is obtained for PY-3 devices with respect to the fill factor is obtained for PY-3 devices with respect to the reference, i.e., 69.5% (PY-3) vs. 59.5% reference, i.e., 69.5% (PY-3) vs. 59.5% (spiro-OMeTAD). The fill factor is related to the series resistance (spiro-OMeTAD). The fill factor is related to the series resistance and shunt resistance, and the and shunt resistance, and the enhanced hole-transporting and electron-blocking abilities of PY-3 HTM enhanced hole-transporting and electron-blocking abilities of PY-3 HTM are responsible for aredecreased responsible for decreased recombination for the photogenerated charges and, thus, higher fill recombination for the photogenerated charges and, thus, higher fill factor. In fact, the factor. In fact, of the introduction the pyrenederivatives core in arylamine results in HTMs with introduction the pyrene core of in arylamine results in derivatives HTMs with an electron-blocking anability electron-blocking ability superior to that of spiro-OMeTAD, while at the same time superior to that of spiro-OMeTAD, while at the same time keeping the keeping synthesisthe synthesis costs lower [55]. costs lower [55].

Figure5.5. Structures Structures of Figure of pyrene-based pyrene-basedHTMs: HTMs:PY-1, PY-1,PY-2, PY-2,PY-3. PY-3.

(b) Truxene-core HTMs (b) Truxene-core HTMs While most of the papers dealing with alternative HTMs to spiro-OMeTAD assume the use of While most of the papers dealing with alternative HTMs to spiro-OMeTAD assume the use of dopants (as for spiro-OMeTAD itself) to achieve competitive efficiencies, Chen et al. have designed a dopants (as for spiro-OMeTAD itself) to achieve competitive efficiencies,(Figure Chen et have designed C3h truxene-core (Trux-I) with OMeTAD terminals and hexyl side-chains 6) al. [57]. Its planar, a Crigid truxene-core (Trux-I) with OMeTAD terminals and hexyl side-chains (Figure 6) [57]. Its planar, 3h and fully-conjugated structure results in an excellent hole-mobility of the pristine material of rigid and fully-conjugated structure results in an excellent hole-mobility of the pristine material −3 2 −1 −1 roughly 10 cm V s , nearly two orders of magnitude higher than that of spiro-OMeTAD and of −1 s−1 , nearly −4 cm2V roughly 10−3 cm2 V(between two10 orders of−1magnitude higher that of spiro-OMeTAD and polytriarylamine 10−5 and s−1 [98]). Indeed, itsthan higher mobility allows one to − 5 − 4 2 − 1 − 1 polytriarylamine 10 PCE and 10 cm V s [98]). Indeed, itsexternal higher mobility allows one to fabricate devices(between with a superb of 18.6%, without introducing any dopants [57]. fabricate devicesGrisorio with a superb PCEsynthesized of 18.6%, without introducing any external [57]. 6) Recently, et al. have Trux-I and a new molecule nameddopants Trux-II (Figure [56]. These star-shaped were designedTrux-I by binding bis(p-methoxyphenyl)amine groups Recently, Grisorio et HTMs al. have synthesized and athe new molecule named Trux-II (Figure 6)to [56]. a truxene-based (Trux-I) by interspacing these functionalitiesgroups from the These star-shapedcore HTMs wereand designed by binding theelectron-donating bis(p-methoxyphenyl)amine to a core with 1,4-phenylene (Trux-II). Thethese authors have subsequently employed them truxene-based core (Trux-I)π-bridges and by interspacing electron-donating functionalities fromasthe HTMs two different perovskite architectures (direct and inverted). Asemployed for the inverted core within1,4-phenylene π-bridges device (Trux-II). The authors have subsequently them as configuration (n-i-p), both HTMs showed a poor performance (PCE = 4.9% and 5% and HTMs in two different perovskite device architectures (direct and inverted). As for for Trux-I the inverted Trux-II, respectively) with respect to spiro-OMeTAD (19.2%). However, in the case of direct device configuration (n-i-p), both HTMs showed a poor performance (PCE = 4.9% and 5% for Trux-I and configuration (p-i-n),with the trend was different: bothHowever, Trux-I- and Trux-II, respectively) respect todramatically spiro-OMeTAD (19.2%). inTrux-II-containing the case of direct cells device outperformed the spiro-OMeTAD reference (PCE = 10.2%, 13.4%, 9.5% for Trux-I, Trux-II and spiroconfiguration (p-i-n), the trend was dramatically different: both Trux-I- and Trux-II-containing OMeTAD, respectively) [56]. The huge difference in the photovoltaic behavior achieved in the two cells outperformed the spiro-OMeTAD reference (PCE = 10.2%, 13.4%, 9.5% for Trux-I, Trux-II configurations depends on the intramolecular charge distributions in radical cations and on the and spiro-OMeTAD, respectively) [56]. The huge difference in the photovoltaic behavior achieved thickness of the HTMs (5–20 nm and 150–200 nm in inverted and direct configuration, respectively). in the two configurations depends on the intramolecular charge distributions in radical cations This study indicates that the performance of PSCs can be effectively tuned by ad hoc device and on the thickness of the HTMs (5–20 nm and 150–200 nm in inverted and direct configuration, architecture modifications. respectively). This study indicates that the performance of PSCs can be effectively tuned by ad hoc device architecture modifications.

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Rakstys et al. have designed and synthesized a series of four two-dimensional triazatruxenebased derivatives (Triazatrux-I, Triazatrux-II, Triazatrux-III and Triazatrux-IV; Figure 6) using inexpensive starting materials and simple synthetic procedures for low production costs [58]. These centrosymmetric star-shaped HTMs, which comprise a planar triazatruxene core and electron-rich methoxy-engineered side arms, interact efficiently with the perovskite surface (a mixed perovskite composition, (FAPbI3 )0.85 (MAPbBr3 )0.15 , was chosen), thus providing better hole-injection from perovskite to HOMO levels of the HTMs, as revealed by the time-resolved photoluminescence studies. The Triazatrux-II-based solar cell exhibited power conversion efficiency of 17.7%, which is slightly higher than that of spiro-OMeTAD device (17.1%) [58]. The triazatruxene-based design guidelines open new paths for constructing low-cost and high-performance hole-transporting materials for PSCs. Rakstys et al. recently designed for the first time a series of star-shaped triazatruxene-based donor-π-acceptor HTMs (Triazatrux-V, Triazatrux-VI and Triazatrux-VII; Figure 6) [59]. When studying their application in PSCs, they observed that Triazatrux-VII led to high PCEs (19%), on par with those of spiro-OMeTAD cells. This exceptionally good performance is attributed to a particular face-on stacking organization of Triazatrux-VII on perovskite (a mixed composition, (FAPbI3 )0.85 (MAPbBr3 )0.15 , was chosen) films, which favors vertical charge carrier transport through an ordered structure. These results are particularly interesting because they represent a unique example of highly-efficient PSCs based on a pristine HTM without any chemical additives or doping. This work paves the way toward the molecular design of next-generation HTMs with high mobility based on a planar donor core, p-spacer and periphery acceptor [59]. (c)

Phenothiazine-based HTMs

The phenothiazine heterocycle plays an important role in the design of high-mobility organic semiconducting materials [99]. Because of their excellent optical, electrochemical and thermal properties, phenothiazine-based sensitizers have been widely used in DSSCs with great performance [100]. Recently, Grisorio and co-workers designed and synthesized two phenothiazinebased molecules, which differ in the aromatic linker (PH-I and PH-II; Figure 7) [60]. PH-I and PH-II were synthesized through straightforward Buchwald−Hartwig and Suzuki−Miyaura cross-couplings, by binding diarylamine or triarylamine groups to a phenothiazine core, respectively. When used as HTM in PSCs, PH-I led to a poor power conversion efficiency of 2.1%, while on the other hand, PH-II exhibited an exceptional PCE of 17.6%, which is close to that obtained with spiro-OMeTAD HTM (17.7%) under the same conditions [60]. The oxidation potential of PH-II (−5.15 eV, which is close to that of spiro-OMeTAD of −5.02 eV) results in high open-circuit voltage (1.11 V for PH-II cells vs. 0.82 V for PH-I devices). Nevertheless, the lower oxidation of PH-I (−4.77 eV) with respect to perovskite (−5.4 eV) is responsible for the more efficient hole-transfer from perovskite to the HOMO level of PH-I. The significantly different photovoltaic behavior of PH-I and PH-II is attributed to the modulation of the electron density distribution, which affects the stability of the molecules during the charge-transfer dynamics at the perovskite|HTM interface. This study demonstrates that, upon minor modifications to the phenothiazine unit, one can achieve significant changes in the PSC performances by low-cost alternatives to spiro-OMeTAD HTM. (d)

Acridine-, thiophene-, biphenyl-, bithiophene-, tetrathiophene- and phenyl-based HTMs

Chao et al. have reported an acridine-based hole-transporting material (AC-I; Figure 8) with a 9,9-dimethyl-9,10-dihydroacridine core, prepared by an easy synthetic procedure consisting of two steps and with good reaction yields [61]. In fact, AC-I does not contain the spirobifluorene motif typical of spiro-OMeTAD, whose preparation requires highly intricate synthetic strategies. Its hole-mobility (in the order of 10−3 cm2 V−1 s−1 , upon doping of additives such as Li-TFSI and tertiary butyl pyridine) is comparable to that of spiro-OMeTAD, and its HOMO level (−5.03 eV) is slightly lower than that of spiro-OMeTAD (4.97 eV). When AC-I was employed as HTM for a perovskite device, a power conversion efficiency of 16.42%, comparable to that of spiro-OMeTAD

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under the same conditions (16.26%), was achieved after HTM thickness optimization (~250 nm), due to Materials 2017, 10, 1087 12 of 44 enhanced charge separation kinetics and recombination resistance. Hence, acridine-based derivatives can be useful low-cost alternatives to spiro-OMeTAD. The synthetic costs of AC-I are estimated to be can be useful low-cost alternatives to spiro-OMeTAD. The synthetic costs of AC-I are estimated to be approximately half of of thethe costs Furthermore,AC-I AC-Ican can synthesized in larger approximately half costsofofspiro-OMeTAD. spiro-OMeTAD. Furthermore, bebe synthesized in larger quantities with a high yield against spiro-OMeTAD [61]. quantities with a high yield against spiro-OMeTAD [61].

Figure 6. HTMs based on a truxene (Trux) and a triazatruxene (Triazatrux) core.

Figure 6. HTMs based on a truxene (Trux) and a triazatruxene (Triazatrux) core.

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Figure Figure 7. 7. Phenothiazine-based Phenothiazine-based HTMs. HTMs.

Liu and co-workers have designed two thiophene-substituted HTMs (Thio-I and Thio-II; Liu and co-workers have designed two thiophene-substituted HTMs (Thio-I and Thio-II; Figure 8), Figure 8), by a simple one-step synthesis of dibromo thiophene with arylamine [62]. The substitution by a simple one-step synthesis of dibromo thiophene with arylamine [62]. The substitution position position of the arylamine moieties on the thiophene π-linker in the two HTMs was connected to the of the arylamine moieties on the thiophene π-linker in the two HTMs was connected to the PSC PSC performance via computational and experimental studies. Thio-II showed better hole-mobility performance via computational and experimental studies. Thio-II showed better hole-mobility than than Thio-I, due to its favorable conjugation in the 2,5 positions as compared to that in the 3,4 Thio-I, due to its favorable conjugation in the 2,5 positions as compared to that in the 3,4 positions of positions of Thio-I. As a result, a good overall solar cell performance of 15.13% in a Thio-II-based PSC Thio-I. As a result, a good overall solar cell performance of 15.13% in a Thio-II-based PSC was achieved, was achieved, which is 40% higher than that obtained with Thio-I-containing HTM. This indicates which is 40% higher than that obtained with Thio-I-containing HTM. This indicates that favorable that favorable geometry of HTMs results in enhanced PSC performance. In the same work, when geometry of HTMs results in enhanced PSC performance. In the same work, when spiro-OMeTAD spiro-OMeTAD was adopted as HTM with a concentration of 20 mg mL−1 under similar conditions, was adopted as HTM with a concentration of 20 mg mL−1 under similar conditions, a surprisingly a surprisingly poor performance (PCE = 8.83%) was achieved. However, when the spiro-OMeTAD poor performance (PCE = 8.83%) was achieved. However, when the spiro-OMeTAD concentration was concentration was increased up to 73 mg mL−1, PCE was enhanced up to 15.63%. − 1 increased up to 73 mg mL , PCE was enhanced up to 15.63%. Pham et al. prepared two easily-attainable, biphenyl-based, low-cost and high-performance Pham et al. prepared two easily-attainable, biphenyl-based, low-cost and high-performance HTMs for PSCs (BPH-I, BPH-II; Figure 8) via conventional Suzuki coupling reactions [63]. HTMs for PSCs (BPH-I, BPH-II; Figure 8) via conventional Suzuki coupling reactions [63]. In particular, In particular, BPH-II-based cells exhibited a PCE of 16.42% (spiro-OMeTAD employed under similar BPH-II-based cells exhibited a PCE of 16.42% (spiro-OMeTAD employed under similar conditions led conditions led to PCE of 16.81%), suggesting that BPH-II could be a good low-cost replacement for to PCE of 16.81%), suggesting that BPH-II could be a good low-cost replacement for spiro-OMeTAD. spiro-OMeTAD. Regarding the device stability, PSCs based on BPH-I and BPH-II retain almost 87% Regarding the device stability, PSCs based on BPH-I and BPH-II retain almost 87% of the initial of the initial performance after 10 days, similar to spiro-OMeTAD devices. performance after 10 days, similar to spiro-OMeTAD devices. Rakstys et al. developed a bithiophene-based derivative (2,2’,7,7’-tetrakis-(N,N’-di-4Rakstys et al. developed a bithiophene-based derivative (2,20 ,7,70 -tetrakis-(N,N 0 -di-4methoxyphenylamine)dispiro-[fluorene-9,40-dithieno[3,2-c:20,30-e]oxepine-6’,9’’-fluorene], BTHIO; methoxyphenylamine)dispiro-[fluorene-9,40-dithieno[3,2-c:20,30-e]oxepine-60 ,900 -fluorene], BTHIO; Figure 8) and studied its performance, stability and crystallography [64]. BTHIO, a novel dispiroFigure 8) and studied its performance, stability and crystallography [64]. BTHIO, a novel dispirooxepine derivative, was prepared by using a simple three-step synthetic procedure and low-cost oxepine derivative, was prepared by using a simple three-step synthetic procedure and low-cost precursors. When adopted as HTM, the corresponding PSC exhibited one of the best reported power precursors. When adopted as HTM, the corresponding PSC exhibited one of the best reported power conversion efficiencies of 19.4%, slightly higher than that of the spiro-OMeTAD reference cell (18.8%) conversion efficiencies of 19.4%, slightly higher than that of the spiro-OMeTAD reference cell (18.8%) under similar conditions. Furthermore, BTHIO shows significantly improved stability when under similar conditions. Furthermore, BTHIO shows significantly improved stability when compared compared to spiro-OMeTAD-based cells. to spiro-OMeTAD-based cells. In a very recent report, Zimmermann et al. designed highly electron-rich tetrathiophene-fused In a very recent report, Zimmermann et al. designed highly electron-rich tetrathiophene-fused HTMs (TETRATH-I–IV; Figure 8), differing from each other with respect to the alkoxy groups HTMs (TETRATH-I–IV; Figure 8), differing from each other with respect to the alkoxy groups (methyl, (methyl, butyl, hexyl and decyl, respectively) [65]. All of these derivatives are easy to synthesize and butyl, hexyl and decyl, respectively) [65]. All of these derivatives are easy to synthesize and to purify. to purify. Moreover, TETRATH-I showed higher thermal stability and performance (PCE = 18.1%), Moreover, TETRATH-I showed higher thermal stability and performance (PCE = 18.1%), comparable comparable to the conventional spiro-derivative. Upon introduction of different alkyl groups, the to the conventional spiro-derivative. Upon introduction of different alkyl groups, the solubility of solubility of the tetrathiophene core increased, but at the same time, the efficiency decreased the tetrathiophene core increased, but at the same time, the efficiency decreased dramatically up to dramatically up to 9.7% (TETRATH-IV). In case of TETRATH-I, the solubility increased by heating 9.7% (TETRATH-IV). In case of TETRATH-I, the solubility increased by heating to 100 ◦ C prior to spin to 100 °C prior to spin coating. The PSC performance remained at a high level after heating, yet when coating. The PSC performance remained at a high level after heating, yet when a similar experiment a similar experiment was conducted for the traditional spiro-derivative, the PCE decreased was conducted for the traditional spiro-derivative, the PCE decreased dramatically already upon dramatically already upon heating to 70 °C. This indicates that tetrathiophene can enable the design heating to 70 ◦ C. This that tetrathiophene can enable the design of thermally-stable and of thermally-stable andindicates low-cost PSCs with high performance [65]. low-cost PSCs with high performance [65]. Chen et al. have reported a simple HTM (3,6-difluoro-N1,N1,N2,N2,N4,N4,N5,N5-octakis(4Chen et al. have reported a simple HTM (3,6-difluoro-N1,N1,N2,N2,N4,N4,N5,N5-octakis(4methoxyphenyl)benzene-1,2,4,5-tetraamine, DFTAB; Figure 8), obtained via one-step synthesis using methoxyphenyl)benzene-1,2,4,5-tetraamine, DFTAB; Figure 8), obtained commercially available precursors [66]. When utilized in a PSC, it gave via riseone-step to a PCEsynthesis of 10.4%using with low hysteresis. When DFTAB was used without additional ionic dopants, the corresponding device achieved a stabilized PCE of 6%. The low cost and easy synthesis render this HTM promising for

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commercially available precursors [66]. When utilized in a PSC, it gave rise to a PCE of 10.4% with low hysteresis. When DFTAB was used without additional ionic dopants, the corresponding device Materials 2017, 10, 1087 14 of 44 achieved a stabilized PCE of 6%. The low cost and easy synthesis render this HTM promising for future large-scale applications, especiallyespecially considering that avoiding willdopants significantly future large-scale applications, considering that additional avoiding dopants additional will enhance the long-term stability of the PSCs. significantly enhance the long-term stability of the PSCs.

Figure bithiophene-, tetrathiophenetetrathiophene- and and phenyl-based phenyl-based HTMs. HTMs. Figure 8. 8. Acridine-, Acridine-, thiophene-, thiophene-, biphenyl-, biphenyl-, bithiophene-,

(e) Triazine-based HTMs (e) Triazine-based HTMs Ko and co-workers synthesized electron-deficient triazine core donor-acceptor HTMs (TRIAZ-I Ko and co-workers synthesized electron-deficient triazine core donor-acceptor HTMs (TRIAZ-I and TRIAZ-II; Figure 9), differing by the spacer group (thiophene or phenyl group) and with and TRIAZ-II; Figure 9), differing by the spacer group (thiophene or phenyl group) and with dimethoxytriphenylamine as the donor moiety [67]. Both TRIAZ-I and TRIAZ-II showed dimethoxytriphenylamine as the donor moiety [67]. Both TRIAZ-I and TRIAZ-II showed hole-mobility hole-mobility similar to that of spiro-OMeTAD. When employed as HTMs in PSCs, TRIAZ-I similar to that of spiro-OMeTAD. When employed as HTMs in PSCs, TRIAZ-I exhibited better exhibited better performance than TRIAZ-II (12.5% and 10.90% efficiencies, respectively) due to a performance than TRIAZ-II (12.5% and 10.90% efficiencies, respectively) due to a higher photocurrent higher photocurrent and fill factor. Under similar conditions, spiro-OMeTAD-based PSC showed a and fill factor. Under similar conditions, spiro-OMeTAD-based PSC showed a PCE of 13.45%. Lim et al. PCE of 13.45%. Lim et al. presented two triazine core star-shaped HTMs (TRIAZ-III and TRIAZ-IV; Figure 10) [68]. They found that TRIAZ-IV exhibited a red-shift in the absorption band, as well as better hole-mobility as compared to TRIAZ-III, due to the presence of the electron-rich indeno[1,2-b]thio moiety. These two materials led to excellent PCEs (13.2% and 12.6% for TRIAZ-III and TRIAZ-IV, respectively), comparable to spiro-OMeTAD (13.8%).

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presented two triazine core star-shaped HTMs (TRIAZ-III and TRIAZ-IV; Figure 10) [68]. They found that TRIAZ-IV exhibited a red-shift in the absorption band, as well as better hole-mobility as compared to TRIAZ-III, due to the presence of the electron-rich indeno[1,2-b]thio moiety. These two materials led to excellent PCEs (13.2% and 12.6% for TRIAZ-III and TRIAZ-IV, respectively), comparable to spiro-OMeTAD (13.8%). Materials 2017, 10, 1087 15 of 44

Figure HTMs. Figure 9. 9. Triazine-based Triazine-based HTMs.

(f) (f) BenzotrithiopheneBenzotrithiophene- and and squaraine-based squaraine-basedHTMs HTMs Ontoria etetal.al.have obtained threethree benzotrithiophene-based HTMs HTMs (BZTR-I,(BZTR-I, BZTR-II BZTR-II and BZTR-III; Ontoria have obtained benzotrithiophene-based and Figure 10) by straightforward cross-coupling reactions between different triphenylamine derivatives BZTR-III; Figure 10) by straightforward cross-coupling reactions between different triphenylamine and benzotrithiophene [69]. These materials, further applied to PSCs, showed PCEs of 16%, 17, derivatives and benzotrithiophene [69]. Thesewhen materials, when further applied to PSCs, showed PCEs and 18.2% for BZTR-I, BZTR-II and BZTR-III, respectively, comparable to the reference spiro-OMeTAD of 16%, 17, and 18.2% for BZTR-I, BZTR-II and BZTR-III, respectively, comparable to the reference (PCE = 18.1%) under The higher performance of BZTR-III isofdue to its is better spiro-OMeTAD (PCE =similar 18.1%)conditions. under similar conditions. The higher performance BZTR-III due conductivity and good alignment of the HOMO level to the perovskite valence band. Along the same to its better conductivity and good alignment of the HOMO level to the perovskite valence band. line, Benito et al. line, haveBenito prepared tri-arm tetra-arm isomers BZTR-V; Figureand 10) Along the same et al. have and prepared tri-arm and (BZTR-IV tetra-arm and isomers (BZTR-IV and studied their optical, electrochemical, photophysical properties and PSC performance [70]. BZTR-V; Figure 10) and studied their optical, electrochemical, photophysical properties and PSC ◦ C, and the corresponding photovoltaic devices show These materials highly stable are up highly to 430 stable performance [70].are These materials up to 430 °C, and the corresponding photovoltaic superiorshow performance with PCE of with 19% for and 18.2% for higherThe efficiency devices superior performance PCEBZTR-IV of 19% for BZTR-IV andBZTR-V. 18.2% forThe BZTR-V. higher achieved with the derivative BZTR-IV may be related to its cis-sulfur arrangement, leading to favorable efficiency achieved with the derivative BZTR-IV may be related to its cis-sulfur arrangement, leading interactions the perovskite structure and better hole extraction. al. have recently to favorablewith interactions with the perovskite structure and better Paek hole et extraction. Paek etdesigned al. have squaraine-based (SQ-H, SQ-OC H ; Figure 10) HTMs, identifying them as excellent light harvesters 6 13 (SQ-H, SQ-OC6H13; Figure 10) HTMs, identifying recently designed squaraine-based them as in PSCs [71]. HTMs exhibited PCEs of 14.74% (SQ-H)PCEs and of 14.73% (SQ-OC 6H 13 ), excellent light These harvesters in PSCs [71]. excellent These HTMs exhibited excellent 14.74% (SQ-H) and comparable to the spiro-OMeTAD reference (PCE 15.33%). The air stability of these materials was 14.73% (SQ-OC6H13), comparable to the spiro-OMeTAD reference (PCE 15.33%). The air stability of also investigated: foralso SQ-H, the PCE dropped only 12% upon 300 h of exposure, for these materials was investigated: for SQ-H, theby PCE dropped only byambient 12% upon 300 h ofwhile ambient SQ-OC6 H13 , there no change in PCE. exposure, while forwas SQ-OC 6H13, there was no change in PCE.

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Figure10. 10.Benzotrithiophene Benzotrithiophene (BZTR)(BZTR)- and Figure and squaraine squaraine(SQ)-based (SQ)-basedHTMs. HTMs.

(g) Fluorene- and spiro-fluorene-based HTMs (g) Fluorene- and spiro-fluorene-based HTMs Rakstys et al. designed a novel bifluorenylidene-based HTM (FL-I; Figure 11) with a lower bandRakstys et compared al. designed a novel bifluorenylidene-based (FL-I; Figure 11)(−5.09 with eV a lower gap (2.41 eV) to spiro-OMeTAD (3.00 eV), yet with HTM a similar HOMO level vs. band-gap (2.41 compared toemployed spiro-OMeTAD (3.00ineV), yet with a similar HOMO level (− 5.09 eV vs. −5.04 eV) [72].eV) When FL-I was as an HTM PSC, it exhibited a power conversion efficiency −5.04 eV) [72]. When FL-I was employed (18.4%). as an HTM in PSC,being it exhibited a power less conversion efficiency of 17.8%, comparable to spiro-OMeTAD However, almost 50-times expensive than of 17.8%, comparable to spiro-OMeTAD (18.4%). However, being almost 50-times less expensive than spiro-OMeTAD, FL-I is an intriguing candidate for future commercial applications of PSCs. As already mentioned, variouscandidate dopants have been commercial used in HTMs in order toofenhance spiro-OMeTAD, FL-I is an intriguing for future applications PSCs. their electrical conductivity, whilevarious at the same time have decreasing stability and increasing cost of the As already mentioned, dopants been the used in HTMs in order totheenhance their resulting device. Based on these considerations, Wang et al. designed a dopant-free HTM (FL-II; electrical conductivity, while at the same time decreasing the stability and increasing the cost of the Figure 11), which constituted both the polytriarylamine unit N-benzene) and spiro-OMeTAD resulting device. Based on these considerations, Wang et al.(i.e., designed a dopant-free HTM (FL-II; [73]. 11), FL-II showed excellent PCE 16.73% with dopants 12.39% without dopants. The Figure which constituted both the of polytriarylamine unit (i.e.,and N-benzene) and spiro-OMeTAD [73]. corresponding values for a spiro-OMeTAD-based device are 14.84% and 5.91%, respectively. FL-II showed excellent PCE of 16.73% with dopants and 12.39% without dopants. The corresponding Focusing on device stability, device Reddy are and14.84% co-workers developed two fluorene-based HTMs (FLvalues for a spiro-OMeTAD-based and 5.91%, respectively. III and FL-IV; Figure 11) using the Suzuki coupling reaction. In their molecular design, spiro-fluorene Focusing on device stability, Reddy and co-workers developed two fluorene-based HTMs (FL-III was end-capped to a terminal fluorine group of two different sides, while carbazole was linked to the and FL-IV; Figure 11) using the Suzuki coupling reaction. In their molecular design, spiro-fluorene third arm of the triphenylamine core [74]. The two materials differ through the cyano-group endwas end-capped to a terminal fluorine group of two different sides, while carbazole was linked to

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the third arm of the triphenylamine core [74]. The two materials differ through the cyano-group end-capping. For both, the HOMO levels were well aligned with spiro-OMeTAD and PEDOT:PSS. capping. For both, the HOMO levels were well aligned with spiro-OMeTAD and PEDOT:PSS. In In addition, addition,both bothexhibited exhibitedhigh highhole-mobility, hole-mobility,long-term long-term stability and good solubility. Due such stability and good solubility. Due to to such attractive characteristics, both have been employed in PSCs (replacing spiro-OMeTAD), as well attractive characteristics, both have been employed in PSCs (replacing spiro-OMeTAD), as well as as in in thethe organic bulk-heterojunction (BHJ; replacing PEDOT:PSS). The reported PCE for FL-III was 17.25%, organic bulk-heterojunction (BHJ; replacing PEDOT:PSS). The reported PCE for FL-III was higher than for the spiro-OMeTAD referencereference (16.67%).(16.67%). In a BHJIncell, a PCE achieved, 17.25%, higher than for the spiro-OMeTAD a BHJ cell,ofa 7.93% PCE ofwas 7.93% was theachieved, referencethe PEDOT:PSS cell yielding an efficiency of 7.74%. reference PEDOT:PSS cell yielding an efficiency of 7.74%.

Figure11. 11.Fluorene-based Fluorene-based HTMs Figure HTMs for forhigh-performance high-performancePSCs. PSCs.

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Tiazkis et al. have systematically studied the structure-property relationship of a series of fluorene-based HTMs (FL-V, FL-VI, FL-VII, FL-VIII and FL-IX; Figure 11) [75]. They found that hole extraction, molecular planarity and charge-transport properties can be tuned by substitution of an aliphatic group to the meta- and para-positions of triphenylamine fragments. The poor performance observed in the case of meta-substitution may be due to the non-favorable geometry, but in the case of para-substitution, the overall PCE fell in the 9%–16.8% range, close to the spiro-OMeTAD reference value (17.8%). Since spiro-OMeTAD-based derivatives are found in many of the best-performing HTMs, Bi and co-workers recently reported about a newly-designed and easily-synthesized spiro-based HTM (spiro-FL-I; Figure 12) with excellent performance (PCE = 19.8%; 20.8% for the spiro-OMeTAD reference) [76]. In addition, devices based on spiro-FL-I showed less hysteresis, excellent reproducibility in device fabrication and better stability under dark and dry conditions. By following a similar design principle, Xu et al. have prepared two 3D-spiro-fluorene-based HTMs (spiro-FL-II, spiro-FL-III; Figure 12) [77]. Spiro-FL-III exhibited better hole-mobility, better film-forming properties, higher solubility and a deeper HOMO level as compared to spiro-OMeTAD, also yielding higher PCE (20.8% vs. 18.8%; the PCE for spiro-FL-II was 13.6%). Spiro-FL-III showed also excellent stability after long-term aging for six months. Malinauskas and co-workers have reported a set of HTMs with di-substituted and tri-substituted benzene, or a di-substituted thiophene core (SPI-BI, SPI-TH and SPI-TRI, respectively; Figure 12) with fluorine as peripheral unit [79]. The hole-mobility of these materials is in the same order as for spiro-OMeTAD, but their advantage is that they are easy to prepare via two steps, using commercially available precursors, with an overall material cost equal to about one fifth that of spiro-OMeTAD. When employed in PSCs, these HTMs exhibited PCE values up to 20%. Liu et al. have recently reported a series of four spiro-fluorene derivatives differing from each other by the para- and meta-substitution of diphenylamine or triphenylamine moieties (SPI-FL-MM-3PA, SPI-FL-MP-3PA, SPI-FL-MM-2PA and SPI-FL-MP-2PA; Figure 12) [80]. Furthermore, these compounds showed high hole-mobility, good solubility, suitable energy levels and efficient hole-extraction combined with electron-blocking capability, which are all positive characteristics of HTMs. In the devices, SPI-FL-MP-2PA exhibited the best photovoltaic performance, with a PCE of 16.8%, slightly higher than that of the reference spiro-OMeTAD (15.5%) under similar conditions [80]. When SPI-FL-MP-2PA was further adopted in mixed FAPbI3 /MAPbBr3 PSCs, an enhanced PCE of 17.7%, comparable to the spiro-OMeTAD reference (17.6%), was achieved. Furthermore, SPI-FL-MP-2PA-based devices possess drastically improved stability compared to spiro-OMeTAD-containing devices, with 90% of initial PCE retained after 2000 h in ambient temperature, while spiro-based devices retained only 20% of initial PCEs under the same time exposure.

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Figure 12. Spiro-fluorene-based HTMs (1). Figure 12. Spiro-fluorene-based HTMs (1).

To overcome the stability drawbacks of HTM additives, Xu and co-workers have designed To overcome the based stability of HTM additives, Xu and co-workers have designed dopant-free HTMs on drawbacks spiro-fluorene derivatives (one-3′,6′-bis(benzyloxy)-N2,N2,N7,N7dopant-free HTMs based on spiro-fluorene derivatives (one-30 ,60 -bis(benzyloxy)-N2,N2,N7,N7tetrakis(4-methoxyphenyl)spiro [fluorene-9,9′-xanthene]-2,7-diamine (XDB), N2,N2,N7,N70 tetrakis(4-methoxyphenyl)spiro [fluorene-9,9 -xanthene]-2,7-diamine (XDB), N2,N2,N7,N7-tetrakis tetrakis(4-methoxyphenyl)-3′,6′-bis(pyridin-2-ylmethoxy)spiro[fluorene-9,9′-xanthene]-2,7-diamine 0 ,60 -bis(pyridin-2-ylmethoxy)spiro[fluorene-9,90 -xanthene]-2,7-diamine (XOP), (4-methoxyphenyl)-3 (XOP), N2,N2,N7,N7-tetrakis(4-methoxyphenyl)-3′,6′-bis(pyridin-3-ylmethoxy)spiro[fluorene-9,9′0 ,60 -bis(pyridin-3-ylmethoxy)spiro[fluorene-9,90 -xanthene] N2,N2,N7,N7-tetrakis(4-methoxyphenyl)-3 xanthene]-2,7-diamine (XMP), and N2,N2,N7,N7-tetrakis(4-methoxyphenyl)-3′,6′-bis(pyridin-4ylmethoxy)spiro[fluorene-9,9′-xanthene]-2,7-diamine (XPP); Figure 012), in which the pyridine group -2,7-diamine (XMP), and N2,N2,N7,N7-tetrakis(4-methoxyphenyl)-3 ,60 -bis(pyridin-4-ylmethoxy)spiro 0 is the pendant to respective HTMs with different positions of nitrogen atomsgroup (para, is ortho meta to [fluorene-9,9 -xanthene]-2,7-diamine (XPP); Figure 12), in which the pyridine the and pendant substitution) [78]. Among these,positions XPP exhibited a PCE of 17.2% without usemeta of external dopants, respective HTMs with different of nitrogen atoms (para, orthothe and substitution) [78]. whichthese, is much than corresponding PCE for cell (5.5%) similar Among XPPhigher exhibited a PCE of 17.2% without thethe usespiro-OMeTAD of external dopants, whichunder is much higher conditions. When PCE planar were considered,cell XPP-based HTMssimilar even yielded a PCE of 19.5% than corresponding forPSCs the spiro-OMeTAD (5.5%) under conditions. When planar without any external dopant. Further studies of steady-state PL and transient photocurrent PSCs were considered, XPP-based HTMs even yielded a PCE of 19.5% without any externaldecays dopant. for these materials revealed much hole-extracting anddecays hole-transporting capabilities than much for Further studies of steady-state PL andbetter transient photocurrent for these materials revealed spiro-OMeTAD, thus explaining the better performance and long-term stability as compared to the better hole-extracting and hole-transporting capabilities than for spiro-OMeTAD, thus explaining the conventional HTM. better performance and long-term stability as compared to the conventional HTM. One of the most interesting examples of organic HTMs is the one recently proposed by One of the most interesting examples of organic HTMs is the one recently proposed by Saliba et al. [39]. They present a novel compound, 2’,7’-bis(bis(4-methoxyphenyl)amino)spiro Saliba et al. [39]. They present a novel compound, 20 ,70 -bis(bis(4-methoxyphenyl)amino)spiro [cyclopenta[2,1-b:3,4-b’]dithiophene-4,9’-fluorene] (FDT; Figure 13), where an asymmetric fluorine0 ]dithiophene-4,90 -fluorene] (FDT; Figure 13), where an asymmetric fluorine[cyclopenta[2,1-b:3,4-b dithiophene core is substituted by N,N-di-p-methoxyphenylamine donor groups. FDT was designed

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Materials 2017, 10, 1087 20 of 44 dithiophene core is substituted by N,N-di-p-methoxyphenylamine donor groups. FDT was designed on the basis of the interesting optoelectronic properties of spiro-cyclopentadithiophene derivatives. on the basis of the interesting optoelectronic properties of spiro-cyclopentadithiophene derivatives. When used in a mesoscopic configuration instead of spiro-OMeTAD, it leads to one of the highest When used in a mesoscopic configuration instead of spiro-OMeTAD, it leads to one of the highest PCEs reported forforsmall-molecule The advantages advantagesofofFDT FDTare arethe the low costs PCEs reported small-moleculeHTMs, HTMs, i.e., i.e., 20.2%. 20.2%. The low costs of of thethe material (~60 $/g), and the possibility of using toluene for dissolving it, instead of the more hazardous material (~60 $/g), and the possibility of using toluene for dissolving it, instead of the more hazardous chlorobenzene results are areparticularly particularlyinteresting interesting because they chlorobenzeneused usedfor forspiro-OMeTAD spiro-OMeTAD [39]. [39]. These These results because they point towards an entire class of new HTMs with high performance by molecular engineering of point towards an entire class of new HTMs with high performance by molecular engineering of thethe FDT core. FDT core.

Figure13. 13. Spiro-fluorene-based Spiro-fluorene-based HTMs Figure HTMs(2). (2).

(h) Carbazole-based HTMs (h) Carbazole-based HTMs Carbazole-based derivatives have attracted much attention as charge-transporting materials for Carbazole-based derivatives have attracted much attention as charge-transporting materials for organic light-emitting diodes (OLEDs), DSSCs and PSCs [101–103]. Their interesting photophysical organic light-emitting diodes (OLEDs), DSSCs and PSCs [101–103]. Their interesting photophysical properties such as intense luminescence and reversible oxidation processes, together with the properties such as intense processes, reasonable synthetic costs, luminescence the versatility and of thereversible carbazole oxidation reactive sites and the together excellent with chargethe reasonable versatility ofnovel the carbazole reactive sites andfor the excellent charge transport synthetic properties costs, justify the the efforts to find solutions for low-cost HTMs PSCs in this class transport properties justify the efforts to find novel solutions for low-cost HTMs for PSCs in this class of compounds [10,40,82–84,86–92]. of compounds A study [10,40,82–84,86–92]. from Wu et al. reports a carbazole-based compound (CA-I; Figure 14) synthesized with A studytwo-step from Wu et al. reports a carbazole-based compound (CA-I; Figure 14) synthesized a simple reaction from inexpensive and commercially available materials [81]. CA-I with has a higher hole-mobility and conductivity thanand spiro-OMeTAD, it leadsmaterials to 12.3% efficiency simple two-step reaction from inexpensive commerciallyand available [81]. CA-Ifor hasPSCs, higher which is comparable to what the authors achieved with spiro-OMeTAD [104].

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hole-mobility and conductivity than spiro-OMeTAD, and it leads to 12.3% efficiency for PSCs, which is comparable to what Materials 2017, 10, 1087the authors achieved with spiro-OMeTAD [104]. 21 of 44 Leijtens et al. have synthesized two simple carbazole-based, lithium salt-free HTMs, differing by et al. have synthesized twoand simple carbazole-based, differingform by of the alkyl Leijtens group (linear or branched; CA-II CA-III, respectively;lithium Figuresalt-free 14) [82].HTMs, The oxidized alkyl group (linear or branched; CA-II and CA-III, respectively; Figure 14) [82]. The oxidized form thesethe materials, when applied in PSCs, produced similar performance to that of spiro-OMeTAD-based of devices, these materials, when in PSCs, produced performance to dopant-free that of doped thus implying thatapplied simple carbazole-based designsimilar can be useful for making spiro-OMeTAD-based doped devices, thus implying that simple carbazole-based design be PSCs. Another example is a carbazole-based HTMs with two-arm and three-arm structures, can connected useful for making dopant-free PSCs. Another example is a carbazole-based HTMs with two-arm and through TPA, phenylene or diphenylene core units (CA-IV, CA-V, CA-VI; Figure 14) [83]. With these three-arm structures, connected through TPA, phenylene or diphenylene core units (CA-IV, CA-V, systems, a PCE as high as 14.79% was obtained. Recently, Wang et al. synthesized novel CA-VI; Figure 14) [83]. With these systems, a PCE as high as 14.79% was obtained. Recently, carbazole-based derivatives (CA-VII, CA-VIII; Figure 15) with a biphenyl core. Undoped CA-VII Wang et al. synthesized novel carbazole-based derivatives (CA-VII, CA-VIII; Figure 15) with a resulted in a core. PCE Undoped of 4.53%,CA-VII close toresulted that ofinspiro-OMeTAD CA-VIII, in turn, showed much biphenyl a PCE of 4.53%, (5.10%). close to that of spiro-OMeTAD (5.10%). poorer efficiency of 0.19%, due to the mismatched HOMO level with respect to the perovskite CA-VIII, in turn, showed much poorer efficiency of 0.19%, due to the mismatched HOMO level with and increased recombination at the interface [84]. respectelectron to the perovskite and increased electron recombination at the interface [84].

Figure 14. Carbazole-based HTMs (1).

Figure 14. Carbazole-based HTMs (1).

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XuXu et et al.al.have CA-X; Figure Figure15) 15)[92] [92]based basedononthe the carbazole-core. havereported reportedtwo twoHTMs HTMs (CA-IX, (CA-IX, CA-X; carbazole-core. These materials had DSSCsusing using(E)-3-(6-(4-(Bis(2’,4’-dibutoxy(E)-3-(6-(4-(Bis(20 ,40 -dibutoxyThese materials hadpreviously previouslybeen beenused used in in solid-state solid-state DSSCs 0 -biphenyl]-4-yl)amino)phenyl)-4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b0 ]dithiophen-2-yl)-2-cyanoacrylic [1,1[1,1’-biphenyl]-4-yl)amino)phenyl)-4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b’]dithiophen-2-yl)-2cyanoacrylic (LEG4)[85]. as aCA-X sensitizer CA-Xhigher cells exhibited PCE (6%) than those Acid (LEG4) as Acid a sensitizer cells [85]. exhibited PCE (6%)higher than those employing CA-IX employing while under thethe same conditions, of thethe performance of the spiro-OMeTAD(4.5%), while CA-IX under (4.5%), the same conditions, performance spiro-OMeTAD-based device was was 5%. The good performance of CA-X in solid-state DSSCsevidence gives strong evidence of 5%.based Thedevice good performance of CA-X in solid-state DSSCs gives strong of its superiority its superiority over spiro-OMeTAD, pointing towards its further use as HTM in PSCs. The over spiro-OMeTAD, pointing towards its further use as HTM in PSCs. The CA-X-based PSC device CA-X-based PSC conversion device exhibited a power conversion efficiency ofreference: 9.8% (spiro-OMeTAD exhibited a power efficiency of 9.8% (spiro-OMeTAD 10.2%). Thereference: superiority 10.2%). The superiority of CA-X as compared to CA-IX might be due to lower reorganization energy, of CA-X as compared to CA-IX might be due to lower reorganization energy, yielding higher yielding higher hole-mobility than in CA-IX. Kang et al. have designed a family of dendritic hole-mobility than in CA-IX. Kang et al. have designed a family of dendritic carbazole-based carbazole-based star-shaped HTMs (CA-XI, CA-XII, CA-XIII; Figure 16) and investigated their star-shaped HTMs (CA-XI, CA-XII, CA-XIII; Figure 16) and investigated their performance in performance in PSCs [86]. The trimeric structures exhibited higher conductivity than the dimeric one, PSCs [86]. The trimeric structures exhibited higher conductivity than the dimeric one, due to due to crystallization during the fabrication process. As HTM, CA-XIII resulted in a PSC with 13% crystallization during the fabrication process. As HTM, CA-XIII resulted in a PSC with 13% efficiency, efficiency, which is comparable to that of spiro-OMeTAD PSCs (13.76%). Recently, Zhang et al. which is comparable to that of spiro-OMeTAD PSCs (13.76%). Recently, Zhang et al. synthesized synthesized two new carbazole-based materials (CA-XIV and CA-XV; Figure 16) using commercially two new carbazole-based materials (CA-XIV and CA-XV; Figure available di-substituted and tri-substituted phenyl derivatives [87].16) Theusing PSCs commercially based on theseavailable HTMs di-substituted and tri-substituted phenyl derivatives [87]. The PSCs based on these HTMs had PCEs had PCEs of 11.4% and 13.1% for CA-XIV and CA-XV, respectively (spiro-OMeTAD of 11.4% and 13.1% for CA-XIV and CA-XV, respectively (spiro-OMeTAD reference: 12.0%). reference: 12.0%).

Figure 15. Carbazole-based HTMs (2). Figure 15. Carbazole-based HTMs (2).

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Daskeviciene et al. have also developed a simple one-step synthesis for obtaining a low-cost HTM (CA-XVI; Figure 16)have [88]. also Such an HTM aled to PSCs with 17.8% efficiency, comparable to the Daskeviciene et al. developed simple one-step synthesis for obtaining a low-cost spiro-OMeTAD device. Simple enamine chemistry for comparable synthesis, low-cost HTM (CA-XVI;reference Figure 16) [88]. Such an HTM led tocondensation PSCs with 17.8% efficiency, to the spiro-OMeTAD reference device. Simplethe enamine condensation chemistry for synthesis, materials and higher efficiency suggest applicability of CA-XVI designs as HTMs low-cost for future materials and higher efficiencyorganic suggestelectronic the applicability CA-XVI as HTMs for future high-performance and low-cost devices.ofChen et al.designs reported a tetra-substituted high-performance and low-cost organic electronic devices. Chen et al. reported a tetra-substituted carbazole-based HTM (CA-XVII; Figure 17) via a three-step synthesis, using low-cost starting materials, carbazole-based HTM[89]. (CA-XVII; Figure also 17) investigated via a three-step low-cost starting with a PCE of 17.81% The authors the synthesis, role of theusing OMeTAD group in the materials, with a PCE of 17.81% [89]. The authors also investigated the role of the OMeTAD group in system, by synthesizing a molecule similar to CA-XVII, but with a different substituent than OMeTAD. the system, by synthesizing a molecule similar to CA-XVII, but with a different substituent than The corresponding photovoltaic device had extremely low performance (PCE close to zero), which OMeTAD. The corresponding photovoltaic extremely low performance (PCE close have to demonstrates the importance of the OMeTADdevice moietyhad in the HTM design. Zhu and co-workers zero), which demonstrates the importance of the OMeTAD moiety in the HTM design. Zhu and prepared a carbazole-based molecular design with a tetra-phenylene core (CA-XVIII; Figure 16) [90]. co-workers have prepared a carbazole-based molecular design with a tetra-phenylene core CA-XVIII imparts good thermal stability and well-aligned energy levels. When used as HTM, (CA-XVIII; Figure 16) [90]. CA-XVIII imparts good thermal stability and well-aligned energy levels. the resulting PSCs exhibited a PCE of 12.4% without dopants, which is close to 14.3% obtained When used as HTM, the resulting PSCs exhibited a PCE of 12.4% without dopants, which is close to with doped spiro-OMeTAD. Recently, Zhu et al. synthesized a series of carbazole derivatives differing 14.3% obtained with doped spiro-OMeTAD. Recently, Zhu et al. synthesized a series of carbazole in derivatives the 2,7 and differing 3,6 positions (CA-XIX, CA-XX, CA-XXI (CA-XIX, and CA-XXII; Figure 17) [91]. of these in the 2,7 and 3,6 positions CA-XX, CA-XXI andOut CA-XXII; materials, the[91]. 2,7-substituted (CA-XX and CA-XIX) showed(CA-XX not onlyand good solubility due to Figure 17) Out of thesederivatives materials, the 2,7-substituted derivatives CA-XIX) showed thenot highly twisted structure, but also high PCEs of 16.74% and 14.92%, respectively [91], the former only good solubility due to the highly twisted structure, but also high PCEs of 16.74% and 14.92%, being even higher for the spiro-OMeTAD reference. On the other hand, the 3,6-substituted respectively [91], than the former being even higher than for the spiro-OMeTAD reference. On the other carbazole-based HTM CA-XXII exhibited aHTM non-promising PCE of 13.3%. hand, the 3,6-substituted carbazole-based CA-XXII exhibited a non-promising PCE of 13.3%.

Figure 16. Carbazole-based HTMs (3). Figure 16. Carbazole-based HTMs (3).

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Wu et al. have recently introduced a carbazole-based HTM including the electron-deficient Wu et al. have recently introduced a carbazole-based HTM including the electron-deficient benzothiadiazole (BT) core (CA-XXIII; Figure 17) [105], which was compared to the previously reported benzothiadiazole (BT) core (CA-XXIII; Figure 17) [105], which was compared to the previously CA-X by Xu et al. [92]. The introduction of a BT unit between biphenyl structures in CA-XXIII reported CA-X by Xu et al. [92]. The introduction of a BT unit between biphenyl structures in effectively theenhanced intermolecular interactions,interactions, increasing the charge transport, hole-mobility CA-XXIIIenhanced effectively the intermolecular increasing the chargethe transport, the andhole-mobility the thermal and stability of the material. Furthermore, CA-XXIII exhibited better charge collection and the thermal stability of the material. Furthermore, CA-XXIII exhibited better charge transportation properties than the CA-X derivative. as HTM in a PSC PCE collection and transportation properties than the When CA-X used derivative. When useddevice, as HTM in =a 16.87% PSC was achieved for CA-XXIII, higher than for spiro-OMeTAD (15.53%). device, PCE = 16.87% was achieved for CA-XXIII, higher than for spiro-OMeTAD (15.53%).

Figure 17. Carbazole-based HTMs (4). Figure 17. Carbazole-based HTMs (4).

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(i) (i) Other Othersmall-molecules small-moleculesHTMs HTMs Zong (NPH-I, NPH-II; NPH-II; Zong and and co-workers co-workers have have proposed proposed two two novel novel binaphthol-based binaphthol-based designs designs (NPH-I, Figure 18), which differ in the aromatic or aliphatic linkage to binaphthol unit [93]. The electrochemical Figure 18), which differ in the aromatic or aliphatic linkage to binaphthol unit [93]. The electrochemical measurements revealed that 5.41 eV 5.39 eV for that the the HOMO HOMO energy energylevels levelsof ofboth bothmaterials materials(− (−5.41 eV and and − −5.39 NPH-I and NPH-II, respectively) are well-aligned with that of perovskite, and PCE values similar to the spiro-OMeTAD reference reference were werereported. reported.LiLietetal.al.have have made two different HTMs by changing made two different HTMs by changing the the π-linker (biphenyl carbazole; OMe-I, OMe-II; Figure18) 18)[94]. [94].PCEs PCEsof of18.34% 18.34% and and 16.14% π-linker unitunit (biphenyl vs.vs. carbazole; OMe-I, OMe-II; Figure were higher efficiency of of OMe-II could be be duedue to its were obtained obtainedfor forOMe-II OMe-IIand andOMe-I, OMe-I,respectively. respectively.The The higher efficiency OMe-II could to −4 cm 2 V2−1−1s−−11 vs. 7.83 × 10 −5 cm 2−1V− 1 s−1 for OMe-I), indicating clearly −4 −5 2 −1 higher hole-mobility (2.26 × 10 its higher hole-mobility (2.26 × 10 cm V s vs. 7.83 × 10 cm V s for OMe-I), indicating clearly the the promise of carbazole as the core unit HTMs. Based thework workby byLiLiet et al. al. [94], [94], Nazim and promise of carbazole as the core unit of of HTMs. Based ononthe co-workers have recently synthesized three low-cost thiazolo[5,4-d]thiazole-based HTMs (Thiazo-I, Thiazo-II Thiazo-II and and Thiazo-III; Thiazo-III; Figure Figure 19) 19) [95]. [95].

Figure 18. 18. Other Other organic organic HTMs. HTMs. Figure

Their appropriate energy levels, tuned to match those of CH3NH3PbI3, render them suitable to Their appropriate energy levels, tuned to match those of CH3 NH3 PbI3 , render them be employed in PSCs, which exhibited a PCE of 10.60%, 4.37% and 8.63% for Thiazo-II, Thiazo-I and suitable to be employed in PSCs, which exhibited a PCE of 10.60%, 4.37% and 8.63% for Thiazo-III, respectively. In particular, Thiazo-II, with a furan unit in the thiazolo[5,4-d]thiazole-core, Thiazo-II, Thiazo-I and Thiazo-III, respectively. In particular, Thiazo-II, with a furan unit in the provides a good interface with the perovskite film, allowing for ultrafast and complete intermolecular thiazolo[5,4-d]thiazole-core, provides a good interface with the perovskite film, allowing for ultrafast hole transfer from the photoexcited perovskite layer. Three novel tetraphenylmethane(TPM)arylamine-based hole-transporting materials (anisole, Ph-TPM, and bulky arylamine side groups DPA or TPMA, DPA-TPM and TPA-TPM; Figure 19) were presented by Liu et al. [96]. PSCs based on

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and complete intermolecular hole transfer from the photoexcited perovskite layer. Three novel tetraphenylmethane(TPM)-arylamine-based hole-transporting materials (anisole, Ph-TPM, and bulky Materials 2017, 10, 1087 26 of 44 arylamine side groups DPA or TPMA, DPA-TPM and TPA-TPM; Figure 19) were presented by Liu these et al. three [96]. novel PSCs HTMs based on these good three PCE novel HTMs good values of 4.62%,whereas 9.33% and achieved values of achieved 4.62%, 9.33% andPCE 15.06% respectively, 15.06% respectively, whereas under similar conditions, spiro-OMeTAD exhibited a PCE of 15.49%. under similar conditions, spiro-OMeTAD exhibited a PCE of 15.49%.

Figure 19. Other organic HTMs (2). Figure 19. Other organic HTMs (2).

Finally, we want to highlight the work by Petrus et al., who have addressed the issue of reducing Finally, we want to proposing highlightathe workwith by 3,4-ethylenedioxy Petrus et al., who have addressed thecentral issue of HTM synthetic costs by material thiophene (EDOT) as the reducing HTM synthetic costs proposing a material with 3,4-ethylenedioxy thiophene core (EDOT-AZO; Figure 19) by [97]. EDOT-AZO was synthesized by a simple one-step Schiff (EDOT) base as the central core (EDOT-AZO; Figure 19) [97]. EDOT-AZO was synthesized by a simple one-step condensation chemistry of amine and aldehyde of EDOT under ambient conditions using Schiff base condensation of amineEDOT-AZO and aldehyde of least EDOT under ambient conditions inexpensive precursors.chemistry Indeed, currently is the expensive HTM ever reported using in the context of PSCs (the cost of currently EDOT-AZO is only 10 $/g). This work demonstrates adopting inexpensive precursors. Indeed, EDOT-AZO is the least expensive HTM how, ever by reported in the simple chemical and cost-effective raw$/g). materials, it hasdemonstrates been possible how, to synthesize a context of PSCs (theprocedures cost of EDOT-AZO is only 10 This work by adopting high-performance material with only water as the byproduct. When EDOT-AZO was employed as simple chemical procedures and cost-effective raw materials, it has been possible to synthesize a HTM in planar material CH3NH3PbI 3 PSCs, it led to a performance (PCE = 11%) comparable to that of high-performance with only water as the byproduct. When EDOT-AZO was employed spiro-OMeTAD-based PSCs (11.9%). as HTM in planar CH NH PbI PSCs, it led to a performance (PCE = 11%) comparable to that of 3

3

3

spiro-OMeTAD-based PSCs (11.9%).

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3.1.2. Polymer-Based Hole-Transporters Figure 20 summarizes the polymeric hole-transporting materials that will be surveyed in the rest of this section. An overview of their photovoltaic performance is also presented in Table 2. Among the polymer-based HTMs, PTAA was the first one tested in PSCs and so far the most efficient, as well. Starting from an earlier work by Seok et al., where PTAA partly infiltrates into a mesoscopic scaffold forming a zig-zag-like structure (the achieved efficiency of the PSC was 12%), an extensive optimization process has been carried out with the utilization of mixed perovskites (MAPbBr3 /FAPbI3 ), leading to a PCE of over 20% [10,12,47]. The superior performance of PTAA-based PSCs arises from the exceptional hole-mobility of PTAA as compared to other polymers (10−2 –10−3 cm2 V−1 s−1 ), as well as from its strong chemical interaction with perovskite. However, the high molecular weight of PTAA does not allow easy infiltration into the pores of the TiO2 scaffold. In addition, PTAA is an extremely expensive material (~2000 $/g [106]). Poly(3-hexylthiophene) (P3HT) and PEDOT:PSS are very well-known conducting polymers for organic photovoltaic applications and have been adopted as HTMs in polymer solar cells [107]. When P3HT was used as HTM in a mixed perovskite solar cell (MAPbI3 /MAPbI3-x Clx ), an increase in the efficiency from 6.7% (for the traditional MAPbI3 perovskite-based cells) [104] up to 13% (for mixed-ion perovskite cells) [108] was obtained. More advanced structures based on P3HT have been recently developed, such as bamboo-structured carbon nanotubes [109] or P3HT-modified carbon nanotube cathodes [110], displaying high efficiency and stability, while keeping the fabrication costs low. PEDOT:PSS has been mostly adopted in inverted planar PSCs with successful outcomes, thanks to the good matching of its work-function with the valence band of the perovskite, good film properties and low-temperature processability. You et al. have reported a 17.1% efficiency in such a PSC [111], while the highest efficiency with PEDOT:PSS (18.1%) so far has been achieved with perovskite and hydrogen iodide (HI) additive [30], which ensured very high quality of the perovskite film. The main well-known drawback of PEDOT:PSS is its hygroscopicity, which limits the chemical stability of PEDOT:PSS-based cells in ambient conditions. Several other polymers have been tested as HTMs in mesoporous PSCs. Recently, Dubey et al. presented a diketopyrrolopyrrole-based polymer (PDPP3T, poly[{2,5-bis(2-hexyldecyl)-2,3,5,6tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl}-alt-{[2,20 :50 ,200 -terthiophene]-5,500 -diyl}]) that did not decrease the PSC efficiency when used to replace the doped spiro-OMeTAD (12.32% for PDPP3T vs. 12.34% for spiro-OMeTAD) [112]. As an additional attractive feature, the PDPP3T-based device yielded slower device degradation. PCE decreased up to 60.6% of its initial value in 172 h, while for spiro-OMeTAD cells, the loss in PCE was almost 83% of its initial value in the same time. Stringer et al. have designed a carbazole-based co-polymer, Poly[N-90 -heptadecanyl-2,7carbazole-alt-5,5-(40 ,70 -di-2-thienyl-20 ,10 ,30 -benzothiadiazole)] (PCDTBT) [113], which is a well-known donor material in organic BHJ cells. Its applicability as an HTM in PSCs after doping by LiTFSI and TBP was investigated, and a PCE of 15.9%, close to that of spiro-OMeTAD reference, was reported. Its good air stability is due to the low-lying HOMO level (−5.45 eV), lower than in spiro-OMeTAD (in the range of −5.0–−5.22 eV). Thus, PCDTBT-doped HTMs have been used in standard (n-i-p) PSC device architectures, but unfortunately, very low power conversion efficiencies (4.2%) have been obtained [114]. In [115], the authors optimized the thickness and the doping level of PCDTBT in the FTO/c-TiO2 /mesoporous TiO2 /(FAPbI3 )0.85 (MAPbBr3 )0.15 / PCDTBT/Au structure, reaching an excellent performance (PCE = 15.9%). These results were comparable with those with conventional spiro-OMeTAD-based PSCs (PCE = 7.4%). In one recent report by Yu et al., a co-polymer based on Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b0 ]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) has been investigated. Upon doping of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) with PCPDTBT, a PCE of 15.1% was reached [115], the highest ever reported for PCPDTBT-based polymers. Zhou and co-workers have recently prepared a hyper-branched carbazole-based polymer (HB-CZ) in one step via the Suzuki coupling reaction and successfully utilized it as a hole-transporting material for a perovskite device [116]. The polymer absorbs in the UV region (which makes it a screen

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against PSCs’ degradation), has high hole-mobility, a deep HOMO energy level (−5.32 eV) and high LUMO energy (−2.42 eV). HB-CZ-based cells exhibited a PCE of 14.07%, which is higher 2017,devices 10, 1087 made up of commercially available HTMs such as P3HT (PCE = 9.05%) 28 of 44 and than Materials reference polycarbazole (PCz) (6.60%). Liu et al. have in turn designed a highly π-extended copolymer HTM exhibited a PCE of 14.07%, which is higher than reference devices made up of commercially available Poly{3,6-dithiophen-2-yl-2,5-di(2-decyltetradecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-thienylenevinyleneHTMs such as P3HT (PCE = 9.05%) and polycarbazole (PCz) (6.60%). Liu et al. have in turn designed 2,5-yl}a (PDVT-10), with an impressive hole-mobility of 8.2 cm2 V−1 ·s−1 and good performance in PSCs highly π-extended copolymer HTM Poly{3,6-dithiophen-2-yl-2,5-di(2-decyltetradecyl)-pyrrolo (PCE[3,4-c]pyrrole-1,4-dione-alt-thienylenevinylene-2,5-yl} = 13.4%) without doping [117], representing one(PDVT-10), of the highest reported for dopant-free with PCEs an impressive hole-mobility 2 −1 −1 polymer-based of 8.2 cm V HTMs. ·s and good performance in PSCs (PCE = 13.4%) without doping [117], representing one

Figure20. 20.Polymer-based Polymer-based HTMs. Figure HTMs.

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Xu and co-workers have reported a novel carbazole-based non-conjugated polymer (PVCz-OMeDAD), which bears a non-conjugated polyvinyl chain and a hole-transporting OMeDAD unit [118]. The material is obtained by free radical polymerization of vinyl monomer using low-cost raw materials with high reaction yields. The OMeDAD moiety enhances the molecule’s hole-transporting capabilities, and a PCE of 16.09% has been reported, which is significantly better than that of the spiro-OMeTAD-based reference cells (9.62%). Gaml et al. have reported the use of benzodithiophene-based polymer poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b0 ] dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)], PBDTT-FTTE, as an HTM in PSCs [119]. After doping PBDTT-FTTE with 3% of diiodooctane, it led to PSCs with similar performance as conventional spiro-OMeTAD-based devices in an inert atmosphere, but with an enhanced fill factor and open-circuit voltage. Diiodooctane-doped PBDTT-FTTE cells exhibited an efficiency of 11.6%, while the PCE of the non-doped PBDTT-FTTE devices was slightly lower (PCE = 10.3%). This highlights very well the importance of diiodooctane doping for PBDTT-FTTE polymers as efficient HTMs. Further research is mandatory to explore novel polymeric HTMs whose absorption lies beyond that of perovskite, to contribute to the external quantum efficiency of the device. In fact, the so-far proposed HTMs mostly function as charge carriers rather than light absorbers. However, polymeric HTMs offer several drawbacks when thinking of commercialization, such as their polydispersity, which results in more difficult characterization with undefined molecular weight, lower purity, batch-to-batch variation and last, but not least, poor infiltration into the nanostructured material [4,40]. Table 2. Overview of the photovoltaic performance (PCE) of the polymeric HTMs presented in this work. For comparison, the performance of reference cells fabricated under identical conditions is also shown. When not otherwise specified, the reference HTM is spiro-OMeTAD. For some polymers (hyper-branched carbazole-based polymer (HB-CZ) and PDVT-10), a different HTM is used for comparison, as indicated in paren thesis. HTM

PCE (%), HTM

PCE (%), Reference

Reference

PTAA P3HT PEDOT:PSS PDPP3T PCDTBT PCPDTBT HB-CZ PDVT-10 PVCz-OMeDAD PBDTT-FTTE

22.1 13.0 18.1 12.32 15.9 15.1 14.07 13.4 16.09 11.6

-

[12] [108] [30] [112] [113] [115] [116] [117] [118] [119]

12.34 16.6 9.05 (P3HT), 6.60 (PCz) 11.28 (PTAA) 9.62 10.3

3.2. Inorganic Hole-Transporting Materials The great majority of research on efficient PSCs has been performed using organic hole conductors, despite their challenging syntheses and high purity requirements. For large-scale industrial applications, inorganic p-type HTMs represent a promising, but quite unexplored alternative to the organic ones. They combine high mobility with low fabrication costs and good chemical stability [120]. However, to date, only a few examples of inorganic HTMs are present in the literature, due to the limited choice of suitable materials [5]. Moreover, the solvents used for inorganic HTMs can partially dissolve the perovskite, thus compromising the device stability. In this section, we will review several inorganic HTMs in PSCs, with a stronger focus on the latest and most significant research results related to mesoscopic PSCs. In fact, in the literature, the reader can find several other good reviews on this topic already [4,5,120–122]. Nickel oxide (NiO) is probably the most widely-studied and most promising inorganic HTM at present, leading to the highest values of efficiency and stability. The interest in NiO lies in the fact

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that it is a very robust, abundant and low-cost material. It has been employed in different device configurations, such as inverted mesoscopic PSCs [123–126], mesoporous carbon electrode-based configurations, such as inverted mesoscopic PSCs [123–126], mesoporous carbon electrode-based PSCs [127–130] and inverted planar PSCs [131–142]. PSCs [127–130] and inverted planar PSCs [131–142]. In the papers reported above, several treatments have been proposed to enhance the quality and In the papers reported above, several treatments have been proposed to enhance the quality and the homogeneity of NiO thin layers, such as sputtering and pulsed-laser deposition [124,125,135], the homogeneity of NiO thin layers, such as sputtering and pulsed-laser deposition [124,125,135], with an optimized PCE of 17.3%. In other examples, NiO has been modified by doping (e.g., Cu [141]) with an optimized PCE of 17.3%. In other examples, NiO has been modified by doping (e.g., or combined with small metals (e.g., Li, Mg) or PEDOT. Among the referenced works on inverted Cu [141]) or combined with small metals (e.g., Li, Mg) or PEDOT. Among the referenced works on planar PSCs, it is worth highlighting the article by Chen et al., where a large-aperture PSC area with inverted planar PSCs, it is worth highlighting the article by Chen et al., where a large-aperture PSC 15% PCE is presented [132]. area with 15% PCE is presented [132]. Within the “fully-printed PSCs”, an attractive concept when thinking of industrial applications, Within the “fully-printed PSCs”, an attractive concept when thinking of industrial applications, NiO has been selected by Wang et al. as HTM with a multiporous-layered structure (PCE = NiO has been selected by Wang et al. as HTM with a multiporous-layered structure 15.03%) [123,129,130]. (PCE = 15.03%) [123,129,130]. Very recently, a low-temperature solution-processed NiOx thin film was first employed as a HTM Very recently, a low-temperature solution-processed NiOx thin film was first employed as a in both inverted (p-i-n) planar and regular (n-i-p) mesoscopic organic-inorganic hybrid PSCs [143]. HTM in both inverted (p-i-n) planar and regular (n-i-p) mesoscopic organic-inorganic hybrid The greatest achievement of this work was to introduce a presynthesized NiOx directly deposited PSCs [143]. The greatest achievement of this work was to introduce a presynthesized NiOx directly on top of the perovskite film (in n-i-p structures) without decomposing the perovskite itself. After deposited on top of the perovskite film (in n-i-p structures) without decomposing the perovskite proper treatment of the NiOx thin film, a promising power conversion efficiency of 15.9% with itself. After proper treatment of the NiOx thin film, a promising power conversion efficiency of 15.9% negligible hysteresis (Table 3) was obtained for inverted planar PSCs, and 11.8% was obtained for the with negligible hysteresis (Table 3) was obtained for inverted planar PSCs, and 11.8% was obtained flexible devices (Figure 21). This work paves the way to all inorganic PSCs, for efficient, stable and for the flexible devices (Figure 21). This work paves the way to all inorganic PSCs, for efficient, stable low-cost devices. and low-cost devices. Table 3. Device performance of the NiOx -based inverted planar PSC measured under simulated Air Table(AM) 3. Device performance Mass 1.5 conditions [143].of the NiOx-based inverted planar PSC measured under simulated Air Mass (AM) 1.5 conditions [143]. Scan Direction (V) Scan Direction VVOC OC (V) Forward Forward Reverse

Reverse

1.03 1.03 1.03

1.03

2

(%) PCEPCE JSC(mA/cm (mA/cm2)) FFFF JSC (%) (%) (%) 20.58 74.7 20.58 74.7 15.8915.89 20.66 74.2 20.66 74.2 15.9015.90

Figure 21. (a) Flexible PSC employing NiOx as HTM; (b) figures of merit of the flexible PSC Figure 21. (a) Flexible PSC employing NiOx as HTM; (b) figures of merit of the flexible PSC (reproduced (reproduced with permission from [143], published by the Royal Society of Chemistry). with permission from [143], published by the Royal Society of Chemistry).

Copper (Cu) has been also successfully employed to make low-cost Cu-based inorganic HTMs, Copper has been successfully employed towith makegood low-cost inorganic HTMs, which can be(Cu) deposited viaalso solution processing methods pore Cu-based filling. whichThe canfirst be deposited via solution processing methods with pore filling. one, explored by Kamat et al., was based on good copper iodide (CuI), with an overall The first one, explored by Kamat et al., was based on copper iodide (CuI), with overall efficiency of over 6% [144]. This modest performance is mainly attributed to thean low VOC. efficiency The most of over 6% [144]. This modest performance is mainly attributed to the low V . The most interesting OC included cupric interesting examples of this class of HTM appeared after a couple of years and oxide examples of this class of HTM appeared after a couple of years and included cupric oxide (CuO), (CuO), cuprous oxide (Cu2O) and copper thiocyanate (CuSCN). These wide-bandgap semiconductors cuprous oxide (Cu2 O) and copper thiocyanate (CuSCN). These wide-bandgap semiconductors have have high conductivity, suitable energy level and good transparency. These properties may represent high conductivity, suitable energy level and goodtandem transparency. These properties may represent a a paradigm shift particularly for perovskite-based solar cells. paradigm shift particularly for perovskite-based tandem solar cells. Cu2O has a narrow band-gap, high hole-mobility, low-cost and it is environmentally friendly [145,146]. Being highly sensitive to the underlying mixture of perovskite precursors and solvents, in [145], the authors deposited Cu2O by reactive magnetron sputtering. The corresponding PSCs show a PCE of 8.93% and relatively good stability of over 30 days in air.

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Cu2 O has a narrow band-gap, high hole-mobility, low-cost and it is environmentally friendly [145,146]. Being highly sensitive to the underlying mixture of perovskite precursors and solvents, in [145], the authors deposited Cu2 O by reactive magnetron sputtering. The corresponding Materials 2017, 10, 1087 31 of 44 PSCs show a PCE of 8.93% and relatively good stability of over 30 days in air. The was reported reported in in 2014 2014 by by Grätzel Grätzel et et al. al. [147]. [147]. The first first remarkable remarkable work work on on CuSCN CuSCN HTM HTM in in PSCs PSCs was A A PSC PSC with with 12.4% 12.4% efficiency efficiency was was presented, presented, with with aa reference reference cell cell without without HTM HTM displaying displaying only only 9% 9% efficiency. The improved performance exhibited with the CuSCN HTM was due to the efficient charge efficiency. The improved performance exhibited with the CuSCN HTM was due to the efficient extraction and collection from thefrom excited (CH3 NH(CH in this to TiO CuSCN, 3 PbI33NH 2 and charge extraction and collection theperovskite excited perovskite 3PbI3case) in this case) to TiO 2 and respectively, and then to the corresponding electrodes. CuSCN, respectively, and then to the corresponding electrodes. Nazeeruddin recently reported Nazeeruddin et et al. al. recently reported aa PSC PSC based based on on the themixed mixed(FAPbI (FAPbI3 )0.85(MAPbBr 3)0.85(MAPbBr33)0.15 )0.15 composition, composition, in in combination combination with with CuSCN CuSCN HTM HTM with with over over 16% 16% PCE PCE and and aa remarkable remarkable monochromatic monochromatic incident incident photon-to-electron photon-to-electron conversion conversion efficiency efficiency of of 85% 85% [148]. [148]. Under Under similar similar conditions, conditions, the the device device without CuSCN exhibited a PCE of 9.5%, mostly attributed to a significant decrease in the short-circuit without CuSCN exhibited a PCE of 9.5%, mostly attributed to a significant decrease in the short2 down 2to 15.64 mA/cm2 ) and open-circuit current (from 21.8 mA/cm voltage (from 1100 mV 1100 downmV to circuit current (from 21.8 mA/cm down to 15.64 mA/cm2) and open-circuit voltage (from 900 mV) (Figure 22). The authors attribute the higher Jsc with CuSCN to the effective charge transfer down to 900 mV) (Figure 22). The authors attribute the higher Jsc with CuSCN to the effective charge between perovskite and CuSCN, followed by the fast hole transport through CuSCN to the to Au. transfer between perovskite and CuSCN, followed by the fast hole transport through CuSCN the Au.

Figure 22. 22. Current-voltage Current-voltage curves curves for for PSC PSC with with copper copper thiocyanate thiocyanate (CuSCN) (CuSCN) HTM HTM and and without without any any Figure HTM (reproduced (reproduced with with permission permission from from [148], [148], published publishedby bythe theAmerican AmericanChemical ChemicalSociety). Society). HTM

A higher PCE has been reported by Jung et al. by using a low-temperature solution-processed A higher PCE has been reported by Jung et al. by using a low-temperature solution-processed CuSCN, with a highly stable crystalline structure [149]. In addition, the authors also demonstrated a CuSCN, with a highly stable crystalline structure [149]. In addition, the authors also demonstrated high stability of CuSCN-based cells as compared to spiro-OMeTAD-based ones. While the PSC a high stability of CuSCN-based cells as compared to spiro-OMeTAD-based ones. While the PSC fabricated with spiro-OMeTAD degraded to 25% of the initial PCE after annealing for 2 h at 125 °C fabricated with spiro-OMeTAD degraded to 25% of the initial PCE after annealing for 2 h at 125 ◦ C in in air under 40% average relative humidity, the CuSCN-based PSCs maintained approximately 60% air under 40% average relative humidity, the CuSCN-based PSCs maintained approximately 60% of of the initial value. This proves that, by using CuSCN as HTM in PSCs, high efficiency and improved the initial value. This proves that, by using CuSCN as HTM in PSCs, high efficiency and improved thermal stability can be simultaneously achieved. thermal stability can be simultaneously achieved. The stability issue of spiro-OMeTAD-based PSCs was addressed also by Di Carlo et al. in their The stability issue of spiro-OMeTAD-based PSCs was addressed also by Di Carlo et al. in their work on reduced graphene oxide (RGO) as HTM in mesoscopic PSCs [150]. A simple and work on reduced graphene oxide (RGO) as HTM in mesoscopic PSCs [150]. A simple and reproducible reproducible method for the graphene oxide reduction is described, and mesoscopic PSCs with RGO method for the graphene oxide reduction is described, and mesoscopic PSCs with RGO are presented, are presented, together with control structures employing spiro-OMeTAD. Excellent endurance together with control structures employing spiro-OMeTAD. Excellent endurance properties (1987-h properties (1987-h shelf-lifetime) were demonstrated for RGO-cells, confirmed by long-term stability shelf-lifetime) were demonstrated for RGO-cells, confirmed by long-term stability tests including tests including light-soaking experiments. Moreover, employing RGO as HTM resulted in light-soaking experiments. Moreover, employing RGO as HTM resulted in considerably lower considerably lower fabrication costs compared to spiro-OMeTAD. fabrication costs compared to spiro-OMeTAD. Other inorganic HTMs for PSCs include the transition metals oxides MoO3 and VOx. In spite of the advantages of nontoxicity (MoO3), air stability (MoO3) and high work-function (VOx), their adoption has not resulted in very efficient devices [122]. In fact, these HTMs have a very poor surface morphology, which makes the quality of the deposited perovskite films extremely low. For these reasons and because no relevant progress has been subsequently reported about MoO3/VOx HTMs in PSCs, we will not discuss them in further detail. The interested reader may refer to [120] or [122] for

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Other inorganic HTMs for PSCs include the transition metals oxides MoO3 and VOx . In spite of the advantages of nontoxicity (MoO3 ), air stability (MoO3 ) and high work-function (VOx ), their adoption has not resulted in very efficient devices [122]. In fact, these HTMs have a very poor surface morphology, which makes the quality of the deposited perovskite films extremely low. For these reasons and because no relevant progress has been subsequently reported about MoO3 /VOx HTMs in PSCs, we will not discuss them in further detail. The interested reader may refer to [120] or [122] for a comprehensive overview of transition metal oxides HTMs. Materials 2017, 10, 1087 32 of 44 3.3. 3.3. Hybrid Hybrid Hole-Transporters Hole-Transporters The The high high flexibility flexibility of of photovoltaic photovoltaic contacts contacts is is an an essential essential prerequisite prerequisite for for bringing bringing PSCs PSCs closer closer to the industrial fabrication, e.g., by R2R printing technology. In addition, in order to to the industrial fabrication, e.g., by R2R printing technology. In addition, in order to produce produce cost-effective ofof vacuum steps in in thethe fabrication andand to cost-effectivesolar solarcells, cells,ititisisnecessary necessarytotoreduce reducethe thenumber number vacuum steps fabrication replace the Au top-electrode with a less expensive material. Furthermore, the migration of Au from the to replace the Au top-electrode with a less expensive material. Furthermore, the migration of Au from back-hole contact through the entire PSC is PSC responsible for relevant performance losses, thus stressing the back-hole contact through the entire is responsible for relevant performance losses, thus even more the need for alternative metallic contacts. stressing even more the need for alternative metallic contacts. To ofof current contacts in PSCs, a novel concept of hybrid HTMHTM has been Toovercome overcomesuch suchlimitations limitations current contacts in PSCs, a novel concept of hybrid has reported in the literature, which relies on carbon nanotubes. The function of the nanotubes in different been reported in the literature, which relies on carbon nanotubes. The function of the nanotubes in architectures ranges fromranges a hole-transporter additive and an interface modifier, to a hole-transporting different architectures from a hole-transporter additive and an interface modifier, to a system, as well as a charge-selective electrode. The inherent resilience, low-cost and stability of hole-transporting system, as well as a charge-selective electrode. The inherent resilience, low-cost nanotubes makes them an ideal candidate replace the traditional in PSCs.contacts in PSCs. and stability of nanotubes makes them an to ideal candidate to replacecontacts the traditional Initially, free-standing films of single-walled and double-walled carbon Initially, free-standing films of single-walled and double-walled carbon nanotubes nanotubes (CNTs) (CNTs) were were tested in mesostructured PSCs by Li and co-workers [151], with a dual-function of hole tested in mesostructured PSCs by Li and co-workers [151], with a dual-function of hole transporter transporter and and cathode cathode at at the the same same time. time. The The devices devices performed performed poorly poorly (PCE (PCE == 6.3%), 6.3%), due due to to the the relatively relatively high high resistance of the nanotube films and to the lack of charge selectivity (Figure 23). The resistance of the nanotube films and to the lack of charge selectivity (Figure 23). The performance performance significantly increasedupon uponintroducing introducing a spiro-OMeTAD layer, which allowed achieving a of PCE of significantly increased a spiro-OMeTAD layer, which allowed achieving a PCE 9.9%. 9.9%. This work demonstrated that it is possible to eliminate the metal top electrode, such as Au or Ag. This work demonstrated that it is possible to eliminate the metal top electrode, such as Au or Ag.

Figure 23. (a) Photo of freestanding CNT film lifted by tweezers; (b) mesoscopic CH3NH3PbI3 Figure 23. (a) Photo of freestanding CNT film lifted by tweezers; (b) mesoscopic CH3 NH3 PbI3 perovskite solar cell with CNT film electrode; (c) top view SEM images of CH3NH3PbI3 perovskite perovskite solar cell with CNT film electrode; (c) top view SEM images of CH3 NH3 PbI3 perovskite substrate before and (d) after CNT transfer; (e) tilted SEM image of CH3NH3PbI3 perovskite substrate substrate before and (d) after CNT transfer; (e) tilted SEM image of CH3 NH3 PbI3 perovskite substrate (blue) partly partly covered covered by by CNT CNT film film (purple) (purple) (reproduced (reproduced with with permission permission from from [151], [151], published published by by the the (blue) American Chemical ChemicalSociety). Society). American

Carbon nanotubes have been also used as spiro-OMeTAD additives in PSCs [152,153], as well Carbon nanotubes have been also used as spiro-OMeTAD additives in PSCs [152,153], as well as as mixed with spiro-OMeTAD, originating a composite HTM [153,154], or wrapped mixed with spiro-OMeTAD, originating a composite HTM [153,154], or wrapped in polymers [82,153]. in polymers [82,153]. Snaith et al. achieved significant thermal/moisture stability improvements in PSCs by replacing the traditional organic HTM with polymer-functionalized (P3HT, PTAA) single-walled CNT embedded in an insulating polymer matrix (polymethylmethacrylate (PMMA)) [155]. The key intuition was that the HTM has a protective effect on the perovskite structure, by shielding it from atmospheric moisture, responsible for quick degradation. This also explains why a non-hygroscopic HTM like the system proposed in [155] (Figure 24) allowed good stability of the photovoltaic devices

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Snaith et al. achieved significant thermal/moisture stability improvements in PSCs by replacing the traditional organic HTM with polymer-functionalized (P3HT, PTAA) single-walled CNT embedded in an insulating polymer matrix (polymethylmethacrylate (PMMA)) [155]. The key intuition was that the HTM has a protective effect on the perovskite structure, by shielding it from atmospheric moisture, responsible for quick degradation. This also explains why a non-hygroscopic HTM like the system proposed in [155] (Figure 24) allowed good stability of the photovoltaic devices while being suitable for printing technology. This work demonstrates that, when designing new HTMs for PSCS, it is crucial to take into account not only the electronic characteristics of the HTM, but also its hydrophobicity/permeability properties. In fact, a non-hygroscopic HTM is highly recommended because of its against thermal stressing and moisture ingress. Materials 2017, 10,resilience 1087 33 of 44

Figure 24. Schematic illustration of the perovskite solar cell with carbon nanotube/polymer composite Figure 24. Schematic illustration of the perovskite solar cell with carbon nanotube/polymer composite as the the non-hygroscopic non-hygroscopic hole-transporting hole-transporting structure, in the the figure figure (reproduced (reproduced with with as structure, referred referred as as HTL HTL in permission from [155], published by the American Chemical Society). permission from [155], published by the American Chemical Society).

Aitola et al. presented an interesting system composed of a single-walled carbon nanotube thin Aitola et al. presented an interesting system composed of a single-walled carbon nanotube thin film with a drop-cast spiro-OMeTAD. Such an SWCNT:spiro-OMeTAD hybrid composite works as film with a drop-cast spiro-OMeTAD. Such an SWCNT:spiro-OMeTAD hybrid composite works as an an HTM-counter electrode (CE) system, i.e., the solar cell does not need an evaporated metal HTM-counter electrode (CE) system, i.e., the solar cell does not need an evaporated metal electrode. electrode. The record efficiency achieved by Aitola et al. by the PSC with HTM-CE was 15.5%, while The record efficiency achieved by Aitola et al. by the PSC with HTM-CE was 15.5%, while the control the control cell with traditional spiro-OMeTAD and the evaporated Au electrode had 18.8% record cell with traditional spiro-OMeTAD and the evaporated Au electrode had 18.8% record efficiency. efficiency. This study proves that a potentially low-cost HTM-CE composite relying on single-walled This study proves that a potentially low-cost HTM-CE composite relying on single-walled CNTs CNTs can be successfully deposited directly onto the perovskite layer (FAPbI3:MAPbBr3 in this case) can be successfully deposited directly onto the perovskite layer (FAPbI3 :MAPbBr3 in this case) of of a PSC and yield very acceptable performance. More recently, high long-term stability at elevated a PSC and yield very acceptable performance. More recently, high long-term stability at elevated temperatures of PSCs was achieved with HTM-CE manufactured by a simple press transfer of the temperatures of PSCs was achieved with HTM-CE manufactured by a simple press transfer of the CNT CNT film on the mixed ion perovskite absorber and by infiltrating the CNT film with film on the mixed ion perovskite absorber and by infiltrating the CNT film with spiro-OMeTAD [156]. spiro-OMeTAD [156]. The experiments conducted at 60 °C in N2 atmosphere and one Sun showed The experiments conducted at 60 ◦ C in N2 atmosphere and one Sun showed only a modest linear only a modest linear efficiency loss for the devices (Figure 25). Their lifetime, defined as the point efficiency loss for the devices (Figure 25). Their lifetime, defined as the point where 20% of the initial where 20% of the initial PCE is lost, has been estimated as 580 h. At the same time, the authors PCE is lost, has been estimated as 580 h. At the same time, the authors fabricated standard PCSs with fabricated standard PCSs with a Au back contact, which exhibited a dramatic and rapid efficiency a Au back contact, which exhibited a dramatic and rapid efficiency loss due to the Au ion migration in loss due to the Au ion migration in the structure [156]. the structure [156].

CNT film on the mixed ion perovskite absorber and by infiltrating the CNT film with spiro-OMeTAD [156]. The experiments conducted at 60 °C in N2 atmosphere and one Sun showed only a modest linear efficiency loss for the devices (Figure 25). Their lifetime, defined as the point where 20% of the initial PCE is lost, has been estimated as 580 h. At the same time, the authors fabricated standard PCSs with a Au back contact, which exhibited a dramatic and rapid efficiency Materials 2017, 10, 1087 34 of 45 loss due to the Au ion migration in the structure [156].

Figure 25. 25. Maximum Maximum power power point point (MPP) (MPP) tracking tracking of of Au Au and and SWCNT-contacted SWCNT-contacted devices devices at at high high Figure temperatures (reproduced published byby John Wiley & Sons, Inc.). temperatures (reproducedwith withpermission permissionfrom from[156], [156], published John Wiley & Sons, Inc.).

4. Concluding Remarks and Future Perspectives In spite of the exciting and rapid development of perovskite solar cells, intense research efforts are still needed in order to overcome the main bottlenecks to their commercialization, namely stability, upscaling and cost-effectiveness. Printing and coating techniques similar to mass production methods (e.g., roll-to-roll fabrication) would allow fabricating large-scale devices, thus providing a huge step towards the successful manufacturing and commercialization of PSC-based technologies. Solution-processable HTMs play a big role in this respect and should be judiciously designed to allow achieving high device efficiencies and at the same time enhanced stability and cost effectiveness compared to the obvious choice, spiro-OMeTAD. In fact, while spiro-OMeTAD is still currently needed to achieve the highest efficiency, it has mediocre hole-mobility (and thus, it requires hygroscopic dopants) and a high cost. Ideally, HTMs should exhibit the following key properties: a. b. c. d.

e.

Appropriate ionization potentials are essential to guarantee efficient transfer of the photoexcited holes from the perovskite layer to the HTM. Low electron affinities are required to avoid electron recombination at the perovskite| HTM interface. HTMs should also be highly transparent, low-cost and present good film-forming properties at the same time, which ensure smooth and pinhole-free coverage. High mobility: Doping of the organic HTMs is in most cases necessary to ensure high cells performance, but it is usually responsible for significant degradation processes compared to pristine HTMs. To enhance the stability, it would be ideal to avoid any dopant to the pristine HTM. This requires the HTM to have high mobility. Research efforts in this direction must continue, starting from the promising achievements about the highly order columnar design concept as a strategy to facilitate the mobility of charge-carriers by face-on arrangement. This was the case of material Triazatrux-VII, which demonstrated how to enhance the hole-mobility to achieve exceptional device performance. If the use of dopants cannot be avoided because of the poor hole-mobility of the pristine HTM, hydrophobic dopants or air-stable additives should be chosen, as alternatives to the highly unstable dopants commonly used at present (LiTFSI, TBP, FK 209).

In the last few years, a big number of novel HTMs has been proposed, mainly based on organic small-molecules. Fewer examples include polymers, organometallic, inorganic and hybrid HTMs.

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In this work, we have reviewed the most recent advances in HTM design and synthesis. In particular, we have looked at this topic from a synthetic chemist’s prospective, highlighting the correlation between the HTM molecular design and the performance of the corresponding PSCs. We believe that in this way, we will provide inspiration to many scientists in the field, to design the next generation of HTMs for low-cost, efficient and air-stable perovskite photovoltaics. In the context of small-molecule HTMs, different synthetic strategies have been presented in the literature, with the idea of simplifying the intricate synthetic steps required for spiro-OMeTAD, which directly affect its prohibitive cost (500 $/g). Many of the recent small-molecule-based HTM designs, mostly containing di- and tri-arylamine groups, successfully performed in PSCs, similarly or even better than spiro-OMeTAD (see Table 1). Trux-II and Triazatrux-VII are promising organic HTMs with enhanced efficiency and stability with respect to spiro-OMeTAD. They are based on truxene and triazatruxene core, respectively, and can ensure highly-efficient cells when used in pristine form. XPP, BTHIO, spiro-FL-III and SPI-FL-MP-2PA are also interesting examples of HTMs with high efficiency and stability when adopted in PSCs. Further investigation and optimization of these designs may be very beneficial for the future commercialization of perovskite technology. At present, the least expensive HTM remains EDOT-OMeTPA (10 $/g), which displays nice performance in photovoltaics, on par with that of spiro-OMeTAD. FDT (60 $/g) is a more recent example of a low-cost HTM with excellent performance in PSCs. Many possibilities to further engineer and tune the FDT-core should still be explored. As for polymers, the most successful HTM remains PTAA, adopted in the structure displaying the current world efficiency record (22.1%). However, PTAA is extremely expensive (2000 $/g). Another high-performance polymeric HTM is PVCz-OMEDAD, which led to devices with a PCE of around 16%. A common drawback to all polymeric HTMs is the limited reproducibility in terms of molecular weight and batch-to-batch variation. This is why most of the research work is dedicated to novel small-molecule HTM designing, due to their low-cost, high reproducibility, easier synthesis and tunable optical and electrochemical properties. Recently, inorganic HTMs have emerged as a valid class of materials to guarantee high device stability. Particularly, NiO seems to be a suitable candidate for high-performance PSCs. Nevertheless, the examples of inorganic HTMs presented in the literature are still limited. Bigger research efforts are required to identify novel materials with suitable chemical and electronic features to ensure high performance PSCs. In the last year, a new concept of hybrid HTMs, based on carbon nanotubes (CNTs), has appeared. Because of their inherent chemical and mechanical stability, CNTs are a promising and more stable alternative to conventional materials and can be successfully used in their pristine form. Nevertheless, the efficiencies of CNT-based perovskite solar cells do not yet compete with other types of HTMs. Further improvements in the power conversion efficiencies could make them a highly competitive and convenient option for highly performing and stable PSCs. In conclusion, it is not possible to identify a unique direction for the design of the perfect HTM. We have highlighted the most promising molecules so far (see Figure 26) and their structure-property relationship in PSCs. However, since most of the reports refer to MAPbI3 perovskite and recently the highest efficiencies have been achieved with mixed ion perovskites, the effectiveness of the different HTMs should be assessed in similar conditions for a fair comparison. The lack of a systematic report and the limited available data do not allow this.

In conclusion, it is not possible to identify a unique direction for the design of the perfect HTM. We have highlighted the most promising molecules so far (see Figure 26) and their structure-property relationship in PSCs. However, since most of the reports refer to MAPbI3 perovskite and recently the highest efficiencies have been achieved with mixed ion perovskites, the effectiveness of the different HTMs should report Materials 2017, 10, be 1087assessed in similar conditions for a fair comparison. The lack of a systematic 36 of 45 and the limited available data do not allow this.

Figure 26. 26. Summary work, together together with with their their cost cost Figure Summary of the most efficient HTMs discussed in this work, (when available). available). (when

Further and deeper understanding is paramount for further rational design, in addition to the Further and deeper understanding is paramount for further rational design, in addition to the typically used trial-and-error approach. In particular, we want to emphasize the role of photophysics typically used trial-and-error approach. In particular, we want to emphasize the role of photophysics and opto-electronics, which can help with understanding how to optimize the charge extraction and and opto-electronics, which can help with understanding how to optimize the charge extraction and recombination processes by fine-tuning of the molecular structures. Computational methods can also recombination processes by fine-tuning of the molecular structures. Computational methods can offer a valid support in modelling the phenomena taking place in HTMs and at their interface with also offer a valid support in modelling the phenomena taking place in HTMs and at their interface perovskite. The tight multidisciplinary collaboration between chemists, physicists and device with perovskite. The tight multidisciplinary collaboration between chemists, physicists and device engineers is the key for future breakthroughs in HTM research for low-cost, scalable and highly engineers is the key for future breakthroughs in HTM research for low-cost, scalable and highly performing PSCs. performing PSCs. Acknowledgments: P.V. acknowledges the Academy of Finland (Decision No. 268672) for funding this research. J.K.S. is thankful to the Fortum Foundation for financial support. Author Contributions: P.V. has structured the paper and wrote Sections 1, 2, 3.2, 3.3 and 4. J.K.S. wrote Section 3.1. A.P. acts as the group leader. All of the authors have commented and reviewed the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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Hole-Transporting Materials for Printable Perovskite Solar Cells.

Perovskite solar cells (PSCs) represent undoubtedly the most significant breakthrough in photovoltaic technology since the 1970s, with an increase in ...
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